It might be difficult to find such a splitter because it has no valid use case. These power cables are there for a reason.
The mainboard slot can provide 75W, as can a 6 pin connector. An 8 pin connector can provide 150W. Obviously compared to an 8 pin connector, a six pin connector has more headroom relatively to its specification, so you sometimes get away with plugging a 6 pin cable into an 8 pin connector (it’s designed to mechanically fit that way).
A GTX 580 officially uses 244W, the C 2050 238W. That’s 726W altogether. Your power supply provides 2150W + 275W + 3*75W = 675W. Not a good idea to power your three GPUs with.
If you really insist on keeping your current power supply and using it for three GPUs, plug the two 6 pin cables into the C2050 and an 8 pin cable into each of th GTX 580. That way each GPU gets 225W - less than needed, but at least close and evenly distributed. Also the 6 pin connectors likely have a bit of headroom, so at least the C2050 hopefully gets enough power to work reliably.
Man, the main selling point of the Tesla GPUs is their supposed higher reliability. If you invest the money for a Tesla, why do you risk that with an undersized power supply? I hope you don’t want to do Cuda development on this machine. The time you may waste hunting down spurious “bugs” can easily outweigh the money saved.