Some thoughts.
- if you want a "true" double crank, you need to buy a new crank.
- if you want a true double crank and all the proper double stuff that goes with it, you also need to buy a double front derailleur and a short-cage rear derailleur. And unless you have an external bottom bracket, you need to buy a new double-specific bottom bracket (shorter spindle = narrower tread).
- if you want the effects of a double without the expense, just don't shift into the smallest ring. Or take the smallest ring off and re-adjust the front derailleur limit screw.
- if the triple outer- and middle rings are not to your liking, you can get larger / smaller rings.
- triple front derailleurs will work with a double (as will long-cage triple rear derailleurs).
- probably forgot something, but someone else will chime in.