What Determines Double or Triple?
 

I have a set of Shimano 105 shifters that can do both double and triple. Is it the shifters or the front derailleur that determines how many chainrings it will shift? Thanks. Also, does it make sense to get a compact FR? I'm thinking about getting a FSA Gossamer Compact and using my Shimano Ultegra FR if possible.

Shifter. You can run your triple shifter on a double.

In the future, ask this type of question in the mechanics forum. You will get better advice.

I have the 105 shifters and use the Gossamer w. no problems. It's the limit screws that determine how many chainrings you can shift.

Triple cranks usually use longer bottom bracket spindles than doubles.