You need inner chainring bolts with spacers, a new front derailleur that can handle a triple crank, and a slightly longer bottom bracket spindle.
The spacer size will depend on the crank - the inner ring should be the same distance from the mid ring as the mid ring is from the big ring. Using a straightedge and a caliper to calculate this would be a good idea.
Going from double to triple usually means a BB a size up is needed. (eg. If your old one is 115mm, the new one will need to be 118mm.)