keeping 3 chickens in the barn all winter??

I'm new to chickens - short background: I have three 6-wk-old "layer" chickens which were "dumped" on me as incubating eggs ... no notice/plan. They hatched within the first 4 days of September, and have been living in a small dog kennel in the basement. Other 9 eggs didn't hatch. My son was starting 50 CornishX in a 6ft round stock tank - they have been moved outdoors, and will be butchered in a few weeks.
We weren't sure of the sex of these 3, and my son was not willing to build a real coop for them. Tried to give them away, etc.
Seems we have 1 cockerel and 2 pullets - good, so decided to keep them, built still no coop will be forthcoming this year.
Today, I moved them into the tank with all new shavings, a low perch, dustbath/grit pan of new "play sand", some fresh cut alfalfa, heat lamp/temp sensor, food/water.
Top is heavy plywood with a hinged, half-moon lid of 1/2-inch hardware cloth in a frame. It was secure for the broilers for 5 weeks - hoping the same going forward.

I feel a HIGH degree of responsibility for animal care! Already sad that they never saw the light of day until today and have been kept too confined. PLEASE help me to make the best of a bad situation.
Will their health suffer with no direct sunlight? The tank is near the rear barn door, but on the direct north side - probably don't want that door open too much when it's 20-below, and not any real sun coming in anyway.
Should I get them a vitamin supplement - don't care about "organic" strictures at this point, though they are getting organic "grower" ration (same as broilers) which is being partially fermented. Should I change their food at some time before the Spring? Can I cut and dry more alfalfa to give them over the Winter? I WILL make sure they have some kind of proper coop come Spring!

TYVM for any and all advice and encouragement to a newb, nervous chicken momma! BTW, they seem to be Austra White with some probable mix - have been great layers for the guy who gave them to me.
 

Wow! I think I will have to respond with questions more than answers this round!

1.

I have three 6-wk-old "layer" chickens

but

They hatched within the first 4 days of September

I'm hoping that what you meant first days of August, so you really mean 6 week.

2. You haven't mentioned predator pressure - day vs night. That really impacts your options.

3.

I feel a HIGH degree of responsibility for animal care!

I agree - and one of the things I think these birds need right away is some exercise. What are the CornishCross contained in? The layers are smaller but more mobile - can they go join the meat birds for a couple hours a day at least?

Story time: My friend has some banties who due to her landlord passing, had to come and live on my farm. One was broody and wouldn't give up. We gave her regular chicken eggs and she hatched 4 on Aug. 16. They were in a tiny rabbit hutch. I have a movable crate made out of those square panels for shelving that is only one panel high, two wide, by about 6 long. It took about 4 days of training, but the group of mom + 4 will run down their ramp (or fly right off it) cross the field about 60 feet with only a little encouragement (there might be a bug or blade of grass to poke at, doncha know?) and return 6-7 hours later. It's the exercise going there and back that I consider as important as the fresh grass. They're safe from day-time predators more or less and they're locked up at night safe from night ones. It's not perfect, but it is adequate at the moment. It gives them exposure to watching for flying predators and prepares them for when they will likely have to free-range to some extent, unless I actually get multiple paddocks built which is on the list.
We also have several groups of moms with ducklings ranging from 1-3 weeks old. They get free-range time while I'm feeding/filling buckets/collecting eggs/moving portable shelters, but when I'm done and need to leave the field, they get locked back up. As they get older, they'll be allowed some free-range time when I'm not there, but we have a pair of geese which offer a bit of protection and a pair of scare crows I intentionally designed to be fairly easy to move around in an effort to keep them effective. Only the three-week-olds were incubator hatched - having a real mom does help a lot.

Hopefully this will give you some ideas of what you can look around for that could be temporary day-time housing that the birds can be trained to go with you - any sort of frame you can through a tarp over might do the job - and get them at least a little exercise so they grow up strong.

Jay, thanks so much for your interest and long reply!
Yeah, they were born in August - and they seemed to have PTSD/"frozen" when I put them in that much larger space, LOL. Though they are settling down now, but their waterer is blue now, not red like they had. Too skittish for me to put their beaks in.
Broilers are way out in Salatin-type "tractors" ... no predator problems ... but Son has refused to put these 3 in there for just the few next weeks - thinks they will be pecked to death.

Don't want to get too much into the sad "soap-opera" around here, but there is NO reasoning with him/very little empathy for animals/only the barest of help for me/very little conversation - lots of snarling from him.
We have already had a hard freeze Labor Day + next 2 nights ... and forecast again for this Weds.

These 3 poor birds are surely going to be stuck in the barn for the Winter, and I'm just trying to do my best for them.

Being in the barn won't do them any harm at all if you have an old stable in there or something else you can partition off they will be fine. I would say that  a 6ft stock tank is to small for my liking, but still way larger than most commercial chickens get. Keep them out of the wind and provide heat until they are fully feathered. Be careful with the heat lamp in the barn, heat lamps and straw can be a horrible combination. Your son is probably right about adding them to the others, if there is a large size difference they may well get eaten, chickens are not nice creatures in that way. They'll want some light so put a decent light on a timer in there to simulate the daylight hours they would be seeing.

Once they have full feathers they can go outside during the day but I wouldn't let them out before that if it's already cold. They should stay on the grower ration for around 15-20 weeks, then as they start to lay switch to a layers pellet, they will like any kitchen scraps you have as well. Because they will be hitting laying age in the middle of winter they may not start until the days get a bit longer in spring.

My chicks that were born around Aug 1 have been outside for the last month free ranging.  They have an adopted mother hen who protected them from the other birds at first.  They're pretty quick and I think they'd be able to get away from any meat bird.  Unless confined in a small area where they couldn't get away.  

I'd think three chickens in a big barn would be a great way to while away the winter in northern WI.  Sunlight is nice but we don't get much in the winter anyway.