NEMA L6 connectors are used with circuits with a maximum specified voltage of 250 V. Supply connections are intended for two-pole, three wire hot-hot-ground circuits with a nominal supply voltage of 240 V or 208 V, depending on phase configuration. The L6 connector does not provide a neutral connection.
L6-20 and L6-30 connectors are commonly found on in-rack power distribution units in countries where the mains supply voltage is greater than 120 V. They are also found in the US for heavy-duty 240 V equipment such as welders, where the higher supply voltage allows a lower current draw. These connectors are thus found where industrial equipment or large power tools are commonplace."
[They should've added that these loads are all 1-phase loads. BTW, the 230VAC power tool is typically one that has a LONG extension cord.
When I was a kid, professional Skillsaws were commonly wired for 230VAC. This deterred theft. (Adjusted for inflation, those saws cost $900 in todays dollars! At that price they weren't even personal tools. The boss owned them.) They had r e a l l y long extension cords that reached all the way across the lot! They could tolerate mist and drizzle like no other.]
BTW, I can't imagine that your PDU is actually 3-phase. THAT would be unusual.
So what you've got is a 3-phase panel and just two of the hots have been wired up to each L6-30 twist-lock receptacle.
If you're that determined, it's a pretty good bet that the third phase is passing through those receptacle boxes. A typical wiring scheme would have A&B then B&C and then C&A all in a sequence along a given wall.
In which case, you'd replace the receptacle and tap the missing hot.
But, I don't think anyone is making 3-phase PDUs. Picking up the third phase provides absolutely no utility to the system efficiency (not a rotary load) while constraining the market for a given PDU design.