newbie requires guidance on small scale food forest/garden

so i've got 3 beds in my small garden and id like to try out the permaculture design with what i have. I've read a lot about food forest gardening but im a bit afraid to put it into action because i don't know much about plant species.

any input is welcome

Bed 1 - Jerusalem artichokes, red clover
Bed 2 - Parsnips & turnips ( and maybe beetroot) with red clover
Bed 3 - Kale or Rhubarb with red clover

as you can probably tell i don't know much about ground cover and herbaceous plants to use so i've right now just got red clover thrown in to provide nitrogen fixing but it wouldn't be very diverse if i only used red clover. Are there any other plants i could use to help support my food crops?. And i can only fit in 3 layers into my garden (Herbaceous, ground cover and root)

Clover is great (hosts nitrogen fixing microbes, as I'm sure you know) but diversity is definitely better.

A few immediate thoughts come to mind:

1. Plant a variety of things and see what grows for you. Some common soil improvers (daikon, for instance) haven't done well for me. I'm also a cheapskate. The end result is, I use things I can get cheaply for soil improving. Often beans and buckwheat from food sources. (Cheaper in big bags and I bought farm seed when getting things started on our land. But that's a different story.) Half a pound of buckwheat groats can cover a lot of area, then frost-kill and compost in place. The flowers are easy to find if you don't want them to re-seed. Beans (pintos or whatever) host nitrogen fixers, grow easily, and will frost kill eventually. There are other possibilities: sweet potato will grow from slips, create lots of organic matter, and die easily from cold. Squash make lots of leafy matter and you get a bunch of seeds included whenever you buy a squash at the store.

2. Seen from a permaculture mindset, weeds aren't inherently bad. You could see what pops up and how cooperative it is. Not everyone likes this approach, but things like creeping charlie make lots of organic matter with little input from you.

3. Talk to other gardeners around your area. You may find a seed-saver and seed-savers usually have extras. On the other hand, it's surprising (for me, anyway) how many organic and even permie-style gardeners don't save and actively trade seeds. The advantages for you would be clear: free seed that is already acclimated to the general conditions of your area. I doubt I'm in your area but I've got so much of some seed I'll send you samples, just because. But I only grow stuff that does well where I am with little work.

If you include information about your location, you may get better information.

Hau Aaron, first off, Chip has given some great information for you to use.

Now for some things that you need to consider first. Is this garden primarily for food growing?
If so then each bed can and should be home to as many plants as you can fit in to it (bio intensive planting).
Red clover is a great start for ground covering and soil nutrient building but you can add brassicas, the buckwheat mentioned by Chip as well as his other suggestions.
Sweet potatoes grow long vines with largish leaves, in our gardens they grow so well that they end up shading other crop plants so we have turned to growing these in large tubs (also easier to harvest this way).
Winter is the best time to plant the buckwheat since it will not interfere with the usual winter crops like Kale, turnip and mustard greens, winter squash, etc.
When spring comes just chop and drop the buckwheat and plant through the resulting buckwheat mulch.
The more diverse your garden plantings are, the better everything will grow, the one thing to make sure of is that you aren't planting allopathic plants next to each other since they will inhibit each other.
The use of some rotting wood crumble in your gardens will have the natural effect of introducing mycorrhizal fungi, always a good thing for your crop plants since they make more nutrients readily available for your crop plant roots to pick up.

I would also recommend you do a lot of perusing of this site, there is more information here than in any three books you could spend good time reading (research is very much a part of following the permie path)

Good luck in your adventure, we are all here to help each other.