Are you asking because you have a crashed A3 and want to use it as a donor?
Anyhow (and keep in mind I've not been involved in kit cars for about 6 years now) I can not think of any kits I've seen that are based on any audi at all, there are a few BMW based ones but the thing you've got to remember is that apart from a few exceptions kit cars are rear wheel drive and A3s are front or four wheel drive but either way the engine is basically mounted the wrong way for most kits!
now of course, of you really want to use a A3 as a donor then the great thing about kit cars is, you build them yourself and if you have the skills you can fit anything you want into them (last one I done ended up having to cut out and move half the chassis cross members to get a V6 and associated gearbox in it :))
I agree with the snags though, you'd be better off asking on a dedicated kit car forum and also getting yourself a subscription to one or two of the kit car magazines and having a read.
If you want to carry on here then can you clarify the reasons why you are looking at using an A3 and what you want to get out of it.