So i did an experiment on what is the best way to cool the raspberry pi 3.
I used three scenarios and used the 'stress' program to stress the pi3's cpu.
a) Without heatsink: it took about 1min:30sec to reach 80c and thermal throttle
b) With just a Heatsink it took around 3min to reach 80c and thermal throttle
c) With a heatsink and a fan pi3 did not thermal throttle and maintained temps around 60c
I thought that It would be OK with just a heatsink, BUT
Even though these were synthetic tests and not the real scenarios. I did try to compile tangram-es and the pi3 quickly reached 81c in around 3min. So what is happening is that due to the lack of sufficient airflow the heatsink only delays the rise in temp.
So until or unless you do not plan to compile stuff on the pi3 and use it for lightweight stuff like kodi or robot controller etc. you should invest in a small fan.