According to USB specs, a USB port will supply 100mA unless a request is made by the connected device to deliver more current..(up to a MAX of 500mA)
2 USB ports can be used to get up to 1 Amp. Reducing to 3.5 Volts is a simple enough process, and would require a low dropout regulator.
Hubs will only supply 100mA max.
Looks like your main problem is going to be getting your USB port to supply more than 100mA and may require a dedicated USB interface chip.
Might be a lot easier to use the 5 volt rail in your PC from a spare Disk drive power connector, and reduce that to 3.5 volt.