I have been doing a lot of research about building Ar's and I have some plans for builds, but am unsure were to go from here.
The builds I want are:
1) .50 Beowulf Carbine w/ red dot sights
2) .300 Blackout Mid-Rifle w/ Scope & quick sights
3) .338 Lapua Magnum Rifle w/ Scope
I would like advise and help with parts & building the rifles.

First off check this subforum out:

ar variants

Secondly you say you have done some research would you mind sharing with us what your research suggests? This will help us know if you are on the "right track" with your builds. Finally what is the intended purpose of each rifle and what is your experience with AR type rifles to this point?

You should also know that ammo for all three of those builds will be expensive, hopefully you reload or can get into it

Good luck with your builds.

I was researching basics of an Ar, building it, and laws and SBR vs Carbine.
I don't have much experience with Ar's but I have heard a lot that building one is better then just buying a stock one.
I have learned the price of the ammo  I plan of using the guns for big game and home defense/SHTF. I would like to get into reloading.

your going to have an easier time building the 300 blk. Since it is based on the standard ar15 the only difference is a .30cal barrel...aka same bolt mags upper receiver lower stock trigger handgaurd ect.

yes this

Would you recommend making the 300 a carbine and the Beowulf the midrifle?

I would not think 300 blk would be a good SHTF gun just because when the time comes the ammo will be way more rare than the standard (aka military) 223/556 but if thats what you really want for a home defense situation I could go either way. More energy out of a middy versus a carbine, less sharp recoil impulse from the middy compared to carbine, but with a carbine you get a better cyclic rate ( but unless you have an auto sear your not gonna notice that) and you can get shorter barrels usually but this also drop velocity. If your going standard 16" 300 blk then I would prefer the middy for better long range accuracy and less sharp recoil impulse and more muzzle energy.

As for the beowolf I dont know anything about that round but since you want to "big game" hunt with it I would say maximize your barrel length and maximize gas length in order to get more muzzle energy and velocity out of that projectile. but like I said I know nothing about that round.  

Personally if I were getting my first AR I would start off with a 556 gun, learn the weapon and see if you like it (of course you will what am i thinking) then upgrade to the more expensive calibers. You can always have one lower and put different uppers on it (at least for 300blk, there are many others I dont feel like listing) for different applications but always have that real SHTF 556 upper waiting to be used in that circumstance

One mans opinion.

All good points I think I will build a 556 carbine. Than a 300 mid if the first one works out. Any other pointers(best places to get parts, ect)?

The longest barrel length you're going to find for a 300BLK is 16" with either carbine or pistol length gas. You're not going to see worthwhile gains from anything longer.

If you plan on running subsonic ammo get a pistol length gas system.  Carbine length can be finicky with subs. Either gas length will work fine for supersonic ammo.

I think with the .300 I will do a carbine and just use supers instead of sub.

I prefer Palmetto State Armory, Primary Arms , Red Barn Armory, and Aim Surplus. There are a lot others but those are just the ones that stick out...Also for an always good priced Pmag check out Bayou Tactical...Their normal prices are the best non sale prices I've found (including adding shipping in) But Primary Arms has a sale just about every 3 weeks (it seems) and they drop their black M3 Pmags down to like $8.99

Also sometimes you can find good deals on accessories - sights, grips slings stocks ect- on amazon but be careful your not buying the airsoft version.