Can you place any three cards in Monopoly Deal, eg 3 properties at a time or 3 action cards?
The three cards can be any combination of properties, actions, and/or money.  From the rules (abbreviated for clarity):

PLAY UP TO 3 CARDS from your hand, onto the table in front of you. You don't have to play any cards if you don't want to. Play your 3 cards in any combination of the following: A, B, and/or C.
A: Put Money/Bank cards into your own Bank
B: Put down Properties into your own collection
C: Play Action cards into the center