Note: I have read the comics.
It seems that during the Thraxa fight, the Viltrumite ship we see later on was nearby. And Lucan says “It’s finished” in the comms before dying, obviously signaling Kregg and the others to come in.
So, my question is why didn’t they just send more Viltrumites in the first place? Why limit it to 3?
Because the are almost extinct
Exactly, so sending in let’s say 5 against a legendary Viltrumite would make more sense than 3.