Tiagra 9 speed triple front with a 10 speed group I want to temporarily swap parts on two bikes for a trip - I currently have a Tiagra 9-speed setup and a bike with Ultegra 6600 double/10 speed. Is it possible to use the triple front shifter from a 9-speed group with the Ultegra group (substituting an Ultegra triple crank and FD that I'd buy) ?
Yes, it will work, but you need to double check shifting range settings — stops — on the front derailleur.

I recommend use second and third positions of shifter for inner and outer rings respectively (not the first and second).

If you will use not only triple front shifter but also a triple front derailleur you must consider that shifting on double crankset with triple derailleur wouldn't be as fine as with double derailleur (because of triple derailleur frame form).