You didn't say which series components you are using so I can't say for sure. You may need a new crank, front shifter, a new long cage derailleur, and a new chain, as well as a new bottom bracket plus cables, housing and tape. Some shifters are double/triple compatible, some are not. Generally your bottom bracket if internal will not have sufficient spindle length for the triple. You said you were thinking of using a 6603 crank (external brackets) but you didn't say what you presently have. 6600 series front derailleurs are for doubles, 6603 for triples, but it has been my experience that the front derailleur will work both ways (usually enough pivot distance that it will work, not as well as a triple designed front derailleur but it works). As for rear derailleur, the long cage will probably be needed to take up the slack in the chain when on the front small chain ring. You want the GS series rear (if sticking with ultegra) not the SS. Chain will definitely need to be longer.

Having said all that if it were me I would go with a compact crank and leave it at that. What in the world do you need a 28 rear and a small chain ring for********** With that gearing you could climb a wall.