I am relacing the Total line P374-1800 stat with the robertshaw RS3210 stat. the technician who was working on my unit did not finish the switch. I have three cables I don't know where to connect. I have black, white and blue cable and the slots available are (B), (L) , (O), and (W2). What combination might work. I am running a PAYNE PH13 R024-C Heat Pump.

You better go find the guy that started this problem for you since he might know where the wires were removed from the old thermostat

OR

You can trace down each wire by color and find there location on the unit then you will know where they go on the new thermostat.

What combination might work. I am running a PAYNE PH13 R024-C Heat Pump.

Only the correct wiring will make it work so any combination would just be guess work and that can damage your unit.

Thanks for the advice. I will track down the wires.