Mild plot-spoilers.

(SPOILER)The first treasure I think you mentioned is the one with the gerridade sitting on it? While you're in the sewers going to save Riddel, go into the room with all the Spearfishers in it and the big waterfall. Run all the way to the lowerleft corner of the screen and run down. You end up near the bug on the chest in the cave in Shadow Forest. Shake the bars and scare the bug away. Go get the treasure later on in the game (it's a Deluge I think).

To get the second one you mentioned, first turn the valve that moves the door in the sewers. (Go from the first room, into the room with the spearfishers, knock down the barrels, go up the stairs, it's in that room I THINK). Then, go back into the spearfisher room, up the ladder and to the back of the KITCHEN. There's a grate there, go down into it and then try to get to the chest.

As for the third one, you have to wait until after you go back to Viper Manor during the Porre Occupation, (and I think General Viper has to be recruited). Go back into the room, and the guard who was there before is gone. Go to the corner where he was standing and mess with the column. There's a switch there that turns off the spray.