ELP consist of a keyboardist, bassist and drummer.
And then of course we have several bands who lack a bass player...The White Stripes, The Black Keys, Sleater Kinney, Yeah Yeah yeahs, Secret Machines....The Doors could count too though Manzerek played bass parts on his keyboard...Most prog bands have ensembles that are uncommon in most rock...And they are rarely 3 piece, though Rush and ELP are among the exceptions.
If i could list 3 piece bands where all the members have done crazy and experiemental things with their instrument, i would say.
Primus
Tool
Rage Against The Machine
ELP
Cream
The Who
Rush
Black Sabbath
Led Zeppelin
Of course i think this would be easier if it werent just 3 piece outfits.