Barrel length?  16 or 18?

If 16 would it be midlength?

hand guard?

Compensator/flash ?

18" rifle is the standard you see most in 3gun matches.  I have one and am building another one.
I started with a 16" midlength and it did a fine job.  If I had to go 16" it would have an intermediate gas.
hand guard are really personal preference.  I prefer slimmer guards that only have rails on top.  SLR, Noveske are what I used in the past.
Muzzle device would need to be a brake of some sort.  I like SJ Titan and Lantac Dragon.

18" with a RIFLE LENGTH gas system.  If you go with a 16", at a minimum get the mid-length gas system.

You want a 1-6x scope with an ILLUMINATED RETICLE/DOT

The mount is not very important; you DO NOT NEED a LaRue (although they are really really nice mounts with repeatable zero).  But for about $75 or so, an Armalite mount/ring combo & some blue Lock Tite works fine.

Ammo, ammo, ammo!  Practice ammo is what you need & what will help you win matches.  That and dry-fire.

FREE FLOAT handguard is a must.  Its one of the biggest contributors to accuracy after barrel selection.

If the barrel is accurate, the ammo need not be special; American Eagle 55 grain should get you the accuracy you need in most guns.  Its 3gun, not a bench-rest shoot.

A brake is common & many work; the JP/Cooley is fine; if money is tight, Jerry Miculek's brake is cheap and very effective.  

The SJC work great!  Loud as hell too.

I'd just go to JP's website and pick one out.

I'd just go to Legal Transfer's website and buy a Stag 3G upper from Pete.

I built my upper using an Anderson slick side upper, UTG Pro slimline keymod rail 15", voodoo innovations lightweight contour rifle length gas barrel, 16.5". WIth a strike industries checkmate brake.

This makes for a super lightweight and flat shooting upper.

Thanks for the input.

I never thought of Stag as that good of a product. I must be wrong

Looking at the suggestions above, looks like you are in the $700 range for a 18" 3 gun upper?

I think it could be done for less than that.  Notice that I suggested a $75 one-piece scope mount and another guy suggested a UTG free-float tube (about $90?)

-those two components WORK, but they do not cost a lot of $$.  

Any brand flat-top upper should work fine (but some people like the "look" of a billet CNC upper and pay $250 just for the stripped upper.  I am not convinced there is much, if any accuracy gain at all).

To me, the place to put your $$ is into a known accurate, reliable barrel.  After that, put some $$ into the type of optic I suggested.  

Most of all:  just start shooting.  Run what you have in a match.  Any match.  Time spent in competition will show you what works & what you can do better.

Thanks, I have a 20" A2 and a 16" Carbine now. All the parts for a couple lowers and a stripped upper so planning my next build

I have run my 20" with good success I just built my 3 gun rifle here a few weeks ago.

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sweet gun!