PrecisionT5600 Workstation doesn't work with more than 2 graphics cards

My Precision workstation does not recognize more than 2 graphics cards.

Here is my configuration without GPUs:

Windows 10 64 bit., bios revision A15.

1 X 8GB PC3-10600r ram

635W PSU

2 X Xeon E5-2680 V1 CPUs

1 X 3TB SATA HDD

No RAID controller

Here are the GPUs I have on hand:

1 X RX 550

2 X RX 580

1 X R9 270

With just one RX 550 and either a RX 580 or R9 270 plugged directly into the board works.

2 RX 580s directly on the board, won't even post

1 X Rx 580 + 1 X R9 270 directly on the board also won't post.

1 X RX 550 + 1 X RX 580 + 1 X R9 270 also won't post, 1 card in an  un-powered riser.

So at this point I thought it was the power supply, not really sure why when the system was only drawing 400 watts(under load) with R9 270 + RX550. So I tried adding a second PSU with the add2psu(https://www.amazon.ca/Add2PSU-Multiple-Power-Supply-Adapter/dp/B009P98Q8U) and powering all the graphics cards off the second PSU.

I tried 1 X RX 580 in powered riser it worked.

Added the R9 270 on another powered riser, windows detected both cards

Added the second RX 580 on another powered riser, now windows only detects 2 X RX580s and the R9 270 is missing.

Added the RX 550 directly on the motherboard, computer won't post.

Added RX 550 on a powered riser, still only detects two RX 580s.

In each test, I would only introduce 1 new graphics card at a time. I've tried putting the graphics cards in different slots. I've tried putting the risers in different slots. It simply won't recognize 3 graphics cards.

Does anyone have any insight into what the problem might be? Or does this board only support 2 Graphics cards?

Thanks!