I've recently looked at this (double to triple or compact). To go from double to triple, it looks like you need to change the whole groupset, depends on what levers etc. you start with, eg. Shimano Ultegra or DuraAce you need to change the STI levers, the cranks, the chain, the bottom bracket, and prob. derailleurs and even the cables. Plus you need some tools, maybe a crank puller and a BB remover, depending on what cranks you've got on the bike already.

If you go to a Shimano compact double, you may only need to buy the crank as it comes with an integrated bottom bracket (you'd have to check if your 'cups' are suitable), and then i've read you only need to raise the front derailleur 5mm. I'm not sure about what tools you'll need.

The compact looks to be a much cheaper option and will produce almost same results as going to a triple. Depending on what cassette you have, you'll lose a gear at the top end, but if you don't go screaming down hills at 60kph on your big chainring regularly, you might not miss it anyway. The gears you get at the bottom end will also depend on what cassette you put with it. You can use whatever you've already got, but a 12-27 cassette will give you the lowest gears for going up hills.

I've only read up on this as I'm seriously thinking of doing this myself, so I'm not sure if I'm right. I'll go from double to compact double and also change to the 12-27 cassette. I could find when I do it, that it's not so straight-forward.

mags...