If you have a double 105 or Ultegra shifter on the bike, how do you tell whether or not it can run a triple? From postings on this forum, some Ultegra shifters will run a double or a triple front derailleur. Does this also go for 105 shifters too? or are they completely separate, you buy a double or buy a triple but they have some that serve a dual purpose?

A double shifter will not work with a triple crank. A triple shifter will work with a double or triple crank.

Sorry, if you wanted to go triple maybe you should look into a compact crank (50-34). Otherwise you need a new left shifter.

If you have a double 105 or Ultegra shifter on the bike, how do you tell whether or not it can run a triple? From postings on this forum, some Ultegra shifters will run a double or a triple front derailleur. Does this also go for 105 shifters too? or are they completely separate, you buy a double or buy a triple but they have some that serve a dual purpose?
9-speed - Tiagra, 105, & Ultegra shifters work with 2x and 3x.

10-speed - Tiagra & 105 work with either 2x or 3x. Ultegra (and DA) is specific. 6600= 2x and 6603= 3x. It has been unofficially reprted that the 6503 work just fine with 2x - YMMV.

TF