As the user manual suggests: if your run OSPi with RPi 3, you should consider plugging in a microUSB cable to power RPi 3 additional using a USB adapter. Because RPi 3 can draw a lot of current that exceeds the design spec of the current batch of OSPi, providing additional USB power will stablize it. I’ve mentioned a few times on the forum that RPi 3 was released after the current batch of OSPi was manufactured, so it’s really not something we could have predicted.
Another work-around which generally seems successful is to solder a wire or 0ohm resistor to bypass the 3R3 (3.3Ohm ) resistor on the lower-left side of the circuit board. This resistor is there to limit the current draw, and you can bypass it to increase the limit.