Triple (3) Reed Harmoniums
Three reeds sound for every key you press, and you hear it in the first chord: more body, more carry, more instrument underneath the voice.
This is the working performer's standard. A two-reed harmonium is entirely enough for a room and for practice. In a hall, over a group, or against a tabla, the third bank is what stops the harmonium disappearing. Stops let you shut banks off, so the same instrument whispers at home and opens up on stage. That asymmetry is the argument for buying up: a bigger instrument can always play smaller, and a smaller one can never play bigger.
Is it harder to pump? With every bank open it drinks more air, yes. Close the stops you are not using and the bellows go light again. In practice you spend most of your time on two banks and open the third when you need it.
Third reed or scale changer first? They solve unrelated problems. Reeds are tone and volume; the scale changer is transposition. Accompanists need the scale changer first. Soloists and singers get more from the third reed.
If you want the absolute ceiling of harmonium tone and can carry it, see Quadruple Reed.