Ok, well first off I suggest you stop any forward progress and research more. Unfortunately you've already wasted money for no real benefit.
Getting a billet lower by itself and then pairing it with a budget upper is mostly a waste of money. There are some benefits to billet AR-15 lower receivers (kuztom features, better fit and finish (from precise machining tolerances), etc.
However, having said that, the benefits that come from machining are most apparent when you match a billet lower with the billet upper from the same manufacturer. Otherwise, you have a lower with a precise dimension that matches nothing, which is why most billet AR receivers are sold as a matching pair. Otherwise you just faced a 2000-grit sanded piece of marble up against a 50-grit sanded piece of concrete. Yah it fits, but kind of a waste.
If I were you, I would trade the billet lower for a cheaper forged one, and use the profit to help pay for the rest of the AR.
As for the 3-gun rifle in general, don't worry too much about parts. Unfortunately most people who start 3gun quit pretty quickly, for lots of reasons. Build the cheapest-nothing special-bare bones $400 AR-15 and use your budget to buy a cheap pistol and cheap shotgun and a case or two of ammo. If you're lucky, you've already got a pistol & shotgun or a friend that will let you borrow one.
Then, once you go to your first 3-gun intro class (lots of competitive ranges hold them), you'll decide whether you even like the game and want to sink money into it.
The good thing about ARs is they're like legos, when you want to upgrade you just pop off the old part and toss on the new one (basic tools may or may not be required/borrowed). Once you have a complete rifle that flings lead, spend your money on ammo and go shoot several matches. Then you can start wasting/spending money on better parts.
Otherwise, internet ninja-Dustin is getting ahead of 3gun shooter-Dustin, and you may spend money on fancy gear that you can't get your money back out of.