What do you have now? A standard road triple with 74mm BCD tiny ring and 130mm BCD middle and outer rings can go down to 24 teeth for the small one, but the middle won't go below 38. Any smaller would make the bolts get in the way of the chain. 110mm BCD, used on road compacts, will go a little smaller, but I don't remember offhand ever seeing a triple with 110mm BCD for the middle and outer rings. They probably exist but I haven't paid enough attention. Otherwise you're into an MTB crankset. If your frame won't allow a clamp-on front derailleur, you might be limited on how small you can go, since the bracket for the other type might be too high for good operation with smaller rings. Our tandem has 24-42-52 and shifts fine and does not drop the chain even when going from 42 to 24 (which is a big jump). Our low is 24/34 which I put on after the first time I climbed an 8-mile-long 10% grade with my wife who's definitely not a climber!