Some have claimed their Raspberry Pi 3 SBCs even get hotter than 100C under load, but I haven't seen quite that extreme with my peak during monitoring being 80~90C. While raspi-config currently doesn't allow overclocking on the Raspberry Pi 3, if you were to OC this quad-core Cortex-A53 SoC you would certainly want at least a passive heatsink if not an active cooler.
Our friends at LoverPi.com have seen the Raspberry Pi 3 get around 100C under load:
Ouch! But fortunately they do have a $5 USD passive heatsink they are selling that fits on the Raspberry Pi 3 as well as the Raspberry Pi 2.
That small passive heatsink is able to drop the SoC temperature almost in half. If you are interested in this $5 heatsink for the Raspberry Pi, you can find it at Amazon.com. LoverPi.com is also sending out an ODROID-C2 so I can run some hands-on comparison tests with it to deliver more data than my brief ODROID-C2 vs. Raspberry Pi 3 benchmarks. It should be fun especially as Pine 64 sent over one of their cheap 64-bit ARM SBCs.