I breed and raise Sebastopol geese (White and Colored), Indian Runner Ducks, Ancona Ducks, Crested Ancona Ducks, East indies Ducks (bantams), Australian Spotted Ducks (Bantams), Mandarin Ducks (Bantam size) Ameraucana chickens, BCM's, SilveruddsBlue/Isbar's, Coturnix quail, Guinea fowl and Peacocks. I try to plan on my chicks and ducklings to hatch a few days before our goslings so that they can all be started in the brooder together and we raise them all together by hand spending a lot of time with our babies as they grow,, but also keeping multiple types of poultry together for as long as we can safely (when we do separate it's generally due to the size differences or changes in feeding requirements.
Solid White bred Sebastopol geese can be sexed at hatch however if there is Any color in their background that no longer applies. I was taught to vent sex my adults last year and I have roughly a 95% success rate. My Ducklings and Goslings my friend was vent sexing for me until I got more comfortable doing it. 3 months is a long time to wait since DNA can be sent in and even with Covid it's still got a Very fast turnaround (2 weeks max) by 3 months old Sebastopols are easily evaluated for Structure, color and feather type quality and curl. Depending upon how the breeder you found raises their goslings 3 months can make a HUGE difference in if they're going to be socialized with people and if they are going to be wary of you and if they're going to be safe with your Ducks as geese go through a Teenager phase where they are going to screw with your Ducks similar to a elementary school bully. They'll sometimes grab and pull feathers or just charge them and depending on the ducks reaction will depend upon how long it lasts and if you are going to need to intervene for 2-4 weeks. Not all Sebastopols are like that though. Ours are raised with chicks and ducklings whenever possible however due to how fast they grow and how clumsy they can be I often separate the majority at night.
Generally in my experience female goslings and ducklings are more vocal and have a lot more to say about everything than the boys lol. However there's always exceptions to that. Goslings are easiest to vent sex between 5 days and 4 weeks, after the first 2-3 weeks you have to change the position your holding them in to make it easier for you and them. Ducklings can be vent sexed at pretty much any age as ducklings although the Mandarins are easiest between 2 weeks and 6 weeks.
It sounds like this breeder is just wanting to keep them all to grow out so they can choose who to keep. If they raise their goslings by hand and not by their parents and spend a lot of time with them especially if they're already raised with ducklings and chicks you will probably be fine. However if they parent raise, raise in large groups (15-20+) and/or keep them in a outbuilding and move them outside as early as possible I'd suggest looking for another breeder.
Here's a list of questions I'd be asking:
Have you visited this breeder?
How socialized were the adult geese? Were they able to walk into the flock without the flock running away?
Did any of the geese approach them? Or did any approach you (non aggressively)?
What color or colors do they have in their flock? Are any Smooth Or Smooth breasted?
How do they hatch their goslings?
How are their goslings raised?
Why are they waiting until the goslings are 3 months old before sexing?
Do they have any previous buyers for references?
If you decide to wait and go with their goslings at 3 months old are they going to be offering a even exchange should one turn out to be a male?
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