Which oscilloscopes can reliably measure losses in an inverter for an AC motor?
Why consider an oscilloscope? 

Why not a power analyser. Newtons4th or Voltech/Tektronic 

There a range specially tailored for inverters &amp; "PWM Mode".  

The PPA5530 from Newton4th can operate in 3 phase 2 wattmeter + phase 3 


  In the 3 phase 2 wattmeter configuration, the voltages are measured
  relative to phase 3. The phase 1 voltage input is connected across
  phase 1 and phase 3, and phase 2 voltage input is connected across
  phase 2 and phase 3, thus measuring phase to phase voltage directly.
  Phase 1 and 2 current inputs are connected normally. There is no need
  to measure the current in phase 3 as phase 3 has no voltage relative
  to itself so the power contribution is zero. In this mode, the neutral
  channel displays the synthesised phase 3 current. PPA55xx “KinetiQ”
  user manual 8-2 The advantage of this connection method is that 3
  phase power can be measured with only 2 wattmeters. This frees up
  phase 3 of a 3 phase instrument to simultaneously measure the power of
  a single phase input (3 phase 2 wattmeter + phase 3 configuration).
  This allows direct measurement of efficiency in a 3 phase motor drive
  or 3 phase inverter application. The frequency reference for the
  independent phase 3 may be selected to be voltage, current, the mains
  line frequency, or the same as phase 1 &amp; 2. In this mode, frequencies
  up to 1kHz can be measured with phase 3.


The PM6000 from Voltech/Tek is a 6 channel power analyser so you can 3 a 3watt meter method on input and output.

This allows you to measure the utility 3phase power draw and the load 3phase power sink.