Piloted by Rei.

The first "eva" as they're called for short is the only eva to have only one eye. The original eva was colored in scheme involving yellow. Which was changed after the unit suffered major damage when an experiment went wrong and the unit went berserk (ejecting Rei's cockpit or "entry tube" as they're called, injuring Rei in the process). Unit 00 was quickly fixed and put into service but actually see's the least amount of action.

Piloted by Sinji.

This eva is the more interesting of the three and seems to be the alfa male of the bunch.  Being loyal to its pilot the unit will act with out power more times than any other eva to preserve the pilots life. *spoiler alert* 

The eva eventually becomes so unpredictable that it is put to use only when the other two are unable to fight off the angels, and is the only eva to keep its first pilot the whole season.

Piloted by Asuka

This eva comes from Germany and is the first "Production model."  True to the eva's heritage the unit acts on its own (but only once) to preserve Asuka (and Sinji's) life.

*spoiler alert*

This eva is not as loyal as unit 01 and eventually rejects Asuka as its pilot.