It sounds like the special line is an OPX from another system. If you have a high enough release, you could treat it as a "networked" system, using the Uniform Dialing Plan. Then you would just Transfer to the 3 digit extension number, and the system would automatically grab the OPX line and dial the digits for you. If those 3 digit numbers conflict with your own extension numbers, you can manipulate them, you dial some other 3 digit number, and the system dials the real one.
Otherwise, you could put the OPX line in its own pool, create virtual extensions for the 3 digit extension numbers, that are Remote Call Forwarded using the pool dial out code for the OPX line instead of the standard 9.