3 playable races, or just one?

You can play as the Advent, Vasari or TEC (humans).

Yes, all three races are playable. Unless you're wondering if you can capture infrastructure and use enemy ships and abilities in game, that's a no.

EDIT: already answered

Alright, thanks for the help. I'm definitely keeping my eye on this game.

isn't Advent are human too, though only change look cooler and powerful then normal human after reseached, I dont know what they reseached when they were normal human?

The advent and TEC may be the same species, but they are two totally separate groups, with different ships, researches, goals, and just about everything else except (maybe) gene pool. I don't know how much the advent have done with genetic manipulation, ya see

Well, you can play as the three different teams but for the most part the only real difference between the teams is the architecture/models of the ships and the voice over/sound clips you hear. Otherwise they are pretty much 90% the same...

Never mind the unique tech trees, vastly unique abilities.. yeah, they're definitely the same other than models and voices.

While the units may have similar roles, the differing stats, research positions, and abilities render a huge difference. Don't forget, abilities have a huge effect in this game, especially as they are (for the most part) designed to be spammed, not used at "precisely the right moment" ala Starcraft.

Eh, I mean I get the basic idea that Team A's 'basic frigates' are slightly better with shields than the other two or Team B's light Carriers get a bonus here... thus each teams specific ships are slightly different.

But they're all the same thing.I didn't beta test so most of my info came from just highlights and previews before playing this so I had the impression that the three teams were going to be VASTLY different from one another.
It would be nice if each team had unique ships that played to their style. Maybe for example, if the Vasari got a planet-siege and light frigate combo ship to emphasize their warlike mentality. Or the TEC might get trade ships that in a pinch and for a cheap amount of metal and/or credits could be turned into "super light" frigates. Stuff like that. Not just "Ok, every team gets a scout, frigate, LRM, flak, siege, colony, light carrier, heavy corvette, 'debuffer' corvette, 'buffer' corvette and caps"
I guess the OP's question has been answered and this is getting out of the scope of this particular thread.

That's the thing. The races have the appearance of similarity, and can even be played the same way.

On the other hand, if you play the Vasari like you play the TEC, someone playing the Vasari like the Vasari are meant to be played, or someone who plays the TEC the way they're meant to be played, will rip you apart.

Just because the units are close doesn't mean that the playstyles are close in any way.