I'm guessing that our OP isn't very far along their PPL and has been looking at some sample questions and wasn't sure how to approach them.

The data supplied in the POH or flight manual will likely have climb rate tabulated for weight, altitude and delta-T(ISA).

At 3000' the standard temperature will be 9°C so the delta(T) will be -9°. For practical purposes you could use -10° as there will probably be tabulated or plotted for 10 degree steps. There may be additional tables or graphs for different configurations of flaps and undercarriage, so make sure you are looking at data relevant to the clean configuration.

The point about interpolation is valid, as strictly you will likely only have figures for ISA and ISA-10° and the value for ISA-9° will be estimated by linear interpolation.

E.g. if you have 1000' ft/min at ISA and 1100 ft/min at ISA-10° then interpolation would estimate 1090 ft/min at ISA-9°. (colder air is more dense so increases the power available and hence the climb rate).