For activities that cost more than 1 action, your actions can be spent over multiple turns in combat as long as Concentration is maintained and you do not perform other actions or reactions in the meantime (unless they are free).
Example Book, the Mage, wants to cast Glacier Strike (3 actions), but only has 1 action left. He’ll spend 1 action on this turn to start casting the spell, and on his next turn spend 2 more to finish casting it. He’s taking a risk, though: he can’t Defend himself and may need to rely on his party to protect him.