Hi,
Any heatsink calculaton document will give this information. Read through them.
RJA is not valid for your situation, because you use a heatsink.
(RJA is for use without heatsink)
RJC is for use with heatsink.
RJC x power_dissipation gives the difference temperature between junction and case.
Calculate this for each device.
Take the max value of those three.
Take TJ - max_vlaue to get max_heatsink_temp at full load. (add some margin for difference temperature between device_temp and heatsink_temp)
Then calculate the difference between TA and max_heatsink_temp = allowed heatsink_temperature_rise.
Heatsink value = allowed heatsink_temperature_rise / total power dissipation.
Klaus