You may need to describe better what you are asking about.
120/240V is _single phase_ service, obtained from a transformer with a single primary coil and a single center tapped secondary coil.
This single phase transformer can be combined in a bank with 1 or 2 other transformers to form 120/240V 'high leg' _delta_ three phase service. The single phase transformer can be larger than the added 'high leg' transformer in order to supply the three phase load.
This sort of delta service is a totally different beastie from 120/208V _wye_ service.
In theory one could use a bank of three transformers, 2 120V transformers and 1 120/240V transformer, to form a bastard hybrid of 120/208 and 120/240. This is probably better described as a 120/208V three phase wye bank, further banked with a 120/240V single phase transformer, where the neutral of the two banks is tied together, and one of the 120V legs of the wye bank is 'merged' to one of the 120V legs of the single phase bank. I've never heard of such a thing actually used anywhere.
Take a look at these links for standard connections: http://electrical-contractor.net/ubb/Forum15/HTML/000054.html http://electrical-contractor.net/ubb/Forum15/HTML/000053.html