When building a squad can you use build from all 3 factions?

No, you pick one faction and build from there, there are some upgrades that can be used across all three factions (mostly crew/EPTs) but you cannot combine factions (unless you're doing a form of the game where you allow ships from different factions to work together)

Thanks. I was seeing some builds that did and it confused me.

What builds?

Perhaps he saw the Captured TIE being used. Thats a specific Rebel ship.

Nott sure where I saw them. I've been Googling my little brain out

Some ships exist in more than one faction:

Scum and Empire share the Firespray

Scum and Rebels share the Z-95, Y-Wing, Hawk

Same ships, but different pilots (and sometimes the presence of an illicit upgrade slot).

23 minutes ago, dotswarlock said:

Some ships exist in more than one faction:

Scum and Empire share the Firespray

Scum and Rebels share the Z-95, Y-Wing, Hawk

Same ships, but different pilots (and sometimes the presence of an illicit upgrade slot).

Maybe that's what it was.

Thanks for the knowledge!

There's always the possibility that you were looking at pure theory "how crazy would it be?" lists, or lists from an event with different squad building rules.