Use the schematic, and work out if what you plan on doing will damage the board. The 2nd page is the generic power block diagram, page 15 is the board power circuits and page 12 shows the USB 3.0 connector.
https://github.com/myriadrf/LimeSDR-USB/blob/master/hardware/socket/1v4/Project%20Outputs%20for%20LimeSDR-USB_1v4s_LMS031pad/LimeSDR-USB_1v4s_schematic_r7.PDF
Running at USB 2.0 data throughput rates (~10x lower than USB 3.0) you will still need to provide 4.5 watts.
You should be able to achieve your maximum data throughput rate over the isochronous USB 2.0 HS (~30 MB/sec at a guess on most USB 2.0 hardware, which could be much much lower on RPi hardware), provided no other USB devices are connected (no keyboard/no mice/nothing at all connected to any USB port) and all communications to/from the RPi3/3+ is using the onboard WiFi (5MB/sec [1 meter away] to 10MB/sec[5 meters away]) and not the USB NIC which is part of the internal 4 port USB hub chip. Point number two here highlights other issues you will have.