The heat generated by the SoC is heavily dependent on the load. Note that idle Raspberry Pi 2/3 will perhaps not need any heat sink even at these temperatures, but will need it at any temperatures with full load. In my experience, when running SysBench (prime numbers) on all cores, the SoC without heat sink will heat up in one to two minutes to the maximum temperature (80 degrees C) , and will start throttling down heavily.
To maintain running the processor at full clock for such loads you need a heat sink, and it better be a big one. For our application we use aluminum plate (part of the case, about 200 cm^2 is the size of the plate), thermally connected to the SoC, and the whole plate gets quite warm to the touch, I would say something like 40C.
AFAIK this is the same behaviour than with Raspberry Pi 2, but it gets a bit less hot than Pi 3.
If you are not planning to run Raspberry at such heavy loads, then perhaps you do not need Pi 2 or Pi 3, take Pi 1 and you will be fine with small heat sink sold specifically for Raspberry Pi.