H Bridge Gate Driver Current Sensing
If you'd take time to read a bit more through the datasheet, you'll find that two pages below, in the Current Sense Amplifiers section, this text appears:

The DRV8305 provides three bidirectional low-side current shunt amplifiers. These can be used to sense the current flowing through each half-bridge. If individual half-bridge sensing is not required, a single current shunt amplifier can be used to measure the sum of the half-bridge current. Use this simple procedure to correctly configure the current shunt amplifiers. [...]

So it's up to you to determine if you want to use three separate sensing resistors, or not.
As for the values for 120 A currents, you may want to look into current transformers for that, or Hall sensors, since the last sentence in that section states that with 5 mOhms resistors:

These values allow the current shunt amplifiers to measure ±33 A across the sense resistor.

Which might suggest that 0.5 mOhm would suffice for your application, if it weren't for the dissipated power: 0.0005*120^2=7.2 W.
In short, if you have reasons to monitor the current through each leg, do so at the expense at three current sensors, oterwise it's just like using one single sensor on the supply. Don't just guide yourself on &quot;what others are showing&quot;, because each &quot;other&quot; datasheet has its own applications in mind. It's all about the current requirements.