#3 is on the bridge where Merryll stands. If you've missed it at the beginning of the game, you can't go back there. #5 is somewhere at the Cliffs. Must be on one of the first maps and you need to jump down on the right side of a rock with pink flowers on it. Might be you need to wear Air Sandals.
Alright, that's what I feared, this instalment seems to have far more missable secret rooms as compared to Millennium 2, and I also blew up the Orc fortress before reading that I didn't need to do so.
Thanks, at least I still have the opportunity to loot the 5th room.