I live in a condo with concrete floors. There is an existing 3" cast iron drain in the floor that was being used for slop basin. I wish to use it for a shower instead. My plumber says that the 3" drain has to be changed to 2" drain to put the pan liner because they don't make shower pan drains for 3" pipe. In order to make the change we will have to remove kitchen cabinets and ceiling sheet rock in the condo below in order to access the P Trap. Thus my question to the experts is 'is there any way to use the current 3" drain for a shower?' Do they make a shower pan drain for that size and/or is there another way to do this easily"

Of course there are. They are called 3" floor drains usually in cast iron.

time to get a better informed plumber..
[www.lowes.com]

He's lying. A 3" flanged drain where he can install pan liner under flange. A 3" drain is better anyways as it is less likely to clog in the future and is easier to snake if it plugs up!