You mean Crossbow Expert, but yes, and you still get the 3 attacks per round a fighter would! Also Three feats, so get your dex up to 20, add Sharpshooter so never disadvantage from high ground...
The only thing I miss here, is that you never get to pick an archery fighting style to offset your -5 to attack. You shot very fast, but oftentimes miss, as well. SO by that rationale, I'd much rather do a 6 Fighter [three feats, 2APR, fighting style] 6 Thief [2 Bonus Actions]
That way you have *four* shots a round, +2 to attack from fighting style and still get 3D6 from Sneak damage.
Either way, thief is the only class that consistently gets 3 Attacks in at Level 3, although a Monk technically attacks three times using one mainhand and a Flurry of Blows. By that token, a Monk 8 / Thief 4 gets the most attacks per round unbuffed; two mainhand, four barehand - that's six. A fighter 11 can do that once per short rest, a thief monk can do it consistenly at level eight.
BUt. While this is all fun. Nothing beats the Water/Lightning combos possible with this, or the allmighty barrelmancer.
Nah I mean Sharpshooter, Crossbow Expert sadly does just about nothing in this game. Sharpshooter is the one that grants +10 damage for -5 attack. You get bonus action attacks while dual wielding crossbows for free as all classes.
Hunter also gets 3 attacks at level 3, whether from the gloom ambush (only in the first round), horde breaker (as long as two enemies are adjacent), or your beast companion (always but harder to pump up damage on it). Also just fwiw I’ve often seen monk builds called “consistent” but it’s really like 4 rounds per short rest, some builds I’ve seen even less, they often gloss over how fast you burn through Ki at 2 points per round on Flurry off Blows.
But yeah, that’s not even close to optimized from a damage perspective, of course, just a demo in that an unpolished thief builds with Sharpshooter destroys the damage output of those without. In practice I’ve found with enough bonuses and high enough dex the miss chance isn’t a problem, especially if you’re always working to grant advantage, which you should be anyway.
If I wanted to grab the extra attack for that build (giving up evasion, at will invisibility, a feat and especially reliable talent is a big ask, but it is more damage output if that’s what you’re focused on) I’d go Ranger over Fighter, get some spells in the mix and either some of the invisibility power with Gloom or a companion, cap at 5 there to retain evasion. Imho either of those would be stronger builds and more fun to play than basically just action surge.
