"Orange Pi 3 includes four USB 3.0 Type-A ports, one USB 2.0 port, and a micro-USB 2.0 OTG (On-The-Go) port with support for power input"
A USB 3 port, --> 6*.150*A*5*V = 4.5*W max power draw in non-charging only mode
A USB 2 port, --> 5*.100*A*5*V = 2.5*W max power draw in non-charging only mode
https://en.wikipedia.org/wiki/USB#Power
"consumers are encouraged to purchase a quality DC 5V 3A charger to power the Orange Pi 3"
3*A*5*V=15*W
4*4.5*W=18*W, 4 USB 3.0 at max power draw in non-charging only mode
You don't have have full power on all USB ports at the same time, and some devices come with their own DC adapter.