Techno itself has different sub genres... like Detroit Techno, Progressive Techno and so on. The so called Techno u hear on the airwaves or at clubs like Music Underground are actually called Euro. Real Techno has very very minimal vocals and alot of repetitive beats and basslines.
Disco takes alot of cue from the 70s. The disco nowadays geared towards Disco house with vocal and house elements thrown in.
Trance has different sub genres too.. progressive trance, hard trance, vocal trance and so on. There may hve elements of vocals thrown in in some productions. Compared to the real Techno.. Trance is more melodic altho they may sound repetitive in some quarters.
Checkout wiki for more info.