What model / speed rather than groupset should be the question, current Ultegra (6800) won't work with current 105 (5700), but the previous generation Ultegra (6700) is 100% interchangeable with 5700.

For changing to a triple, would look closely at the gear ratios & what gain if any you are going to get, I have a '11 Vaya (the first gen) and can't justify changing to a triple from how I initially built it with a 50/34 (5700) & 11-32 (Sram as mention above) as the gain in gears would be so minimal for the additional cost.