My first suggestion if you know nothing about the AR-15 is to SLOW DOWN!!!
Buy or build JUST ONE upper and lower for now.
Being your first AR-15, my recommendation is to always start with the .223. Once you have more experience then move on to additional uppers and lowers.
This way you don't end up being unhappy with all of your uppers and lowers.
I'll echo this quite loudly if its your first AR build without any previous AR knowledge and experience.
Start with a good brand lower (or buy 2 together since it might save you on shipping and background check). Buy a good quality lower parts kit from Brownells (Spikes, RRA, etc).
You also need to factor in the price of tools and work equipment like a good table, vice block, wrenches, etc. That will add a couple of hundred bucks to the budget of the build.
Good luck and have fun. Building, or rather Assembling AR's is a fun way to know your rifle inside out, but can also be challenging to a beginner. If you really had $3k available now to start, I suggest buying a complete 5.56/.223 AR first and work/build your way up to the other calibers.