Will the 3.3V regulator produce 3.3v when the Arduino is powered from the USB port?
Will the 3.3V regulator on an Arduino Leonardo work when powered from USB?
Yes, provided that
it is an "official" Leonardo or a competent clone and
as long as you do not load the power supply excessively with external loads.
The USB supply feeds the 3V3 regulator via MOSFET T1 at the bottom of the sheet and U3, an LP2985 LDO regulator. If your board has the same circuitry as this it should provide 3V3 from USB.