I have a 3" PVC pipe with the heatpump lineset and thermostat wire in it. I want to know if it would be OK to run a electrical wire through this pipe to add a line to a tankless water heater?

Not sure why you'd want to do this. You'll need a 220 cable which can be purchased fully insulated. If you want to see a flurry of response just start asking, in the plumbing forum, the pros and cons of a tankless water heater.

I have a 3" PVC pipe with the heatpump lineset and thermostat wire in it. I want to know if it woudl be ok to run a electrical wire through this pipe to add a line to a tankless water heater?
I want to do this because its not practical to run a new wire through the wall or ceiling. I want a tankless heater to give me more shop space!

Is this PVC just a chase in the wall to allow the passage of the copper lines? If so you should not put your 220 in the same chase. Those copper lines vibrate and if it caused a breach in the 220 insulation you will get a mineral oil fire and a dead short.

I have a 3" PVC pipe with the heatpump lineset and thermostat wire in it. I want to know if it woudl be ok to run a electrical wire through this pipe to add a line to a tankless water heater?
Its about a 25 ft span

All cables in one raceway need to be rated for the highest voltage that will be in that raceway. So the thermostat cable will need to be rated for at least 250 volts.