I have a 3 speed single piece crank and would like to convert to a three piece. What are the possiblities?

The options are endless, BUT, not the right thing. The cost of doing it would be prohibitive given the value of the bike. Best case new you are looking new Bottom Bracket (20-30) and new crank (75-150) prices given for about bottom end stuff. The other issue is single piece cranks, in my experience, have odd chain lines that do not conform to the standards.