Take a screenshot and draw black boxes over private information. Seeing a drawing that shows the essence of the issue in question, is the most important part.
If the switchgear is MLO and if I understand you correctly (that the three 2000A sources are PV), the limiting factors would be the ampacity of the service conductors and the kVA of the utility transformer, and 705.12 would not apply because the PV connections are supply side and governed by 705.11. But under the 2023 NEC this would not be allowed.
What is on the other side of the transformers from the switchgear? What is MDC? Is the switchgear MLO or is it fed by a disconnect with OCP? We can't help you without a more complete description of what you are trying to do.
705.2(B)(3)(1) is just one of several options to qualify equipment for backfeed, so it doesn't stop you just because you can't use it. However the only option I think might work for you is (B)(3)(5), 'engineering supervision'.
From what I can glean it looks like a supply side connection to a MLO panel.
From what I can glean the utility paralleled three transformers to power it, and the (3) 2000A breakers are the service disconnects, so the busbar is on the load side of the service disconnects. So no, not an MLO. Unusual arrangement AFAIK, but permitted by 230.2(C).
OK, I see that now. So how can he comply with 705.12?
As I said, I think the only option might be 705.12(B)(3)(5), 'engineering supervision'.
I have seen multiple feeds to a bus a couple of times, but there is no reason why load breaker ratings couldn't exceed the bus rating.
The drawing says 3500 Amps.
But the OP said it was 6000A
I don't believe there is any requirement to protect the bus at its rating by an OCPD for a switchboard or switch gear.
If you say so, but that doesn't seem right to me.
