Raspberry Pi 3 Model B retains the same footprint as the older Pi 2 albeit some minor changes. Pi 3 board fits most of the Pi 2 cases but pay attention to these two items.
The PWR and ACT LEDs have moved to the other side of the SD card slot and replaced by WiFi/BT antenna. So when using Pi3 with a Pi2 case with light guide, the LEDs are not visible.
With the introduction of push-pull SD card slot, some Pi 2 cases do not have enough clearance to support this mechanism. So it will be slightly difficult to remove the micro SD card in some cases.