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@Majestad 

SLI is not supported on those GPU and the 7910 is not a mining rig so its not likely to work with 3 gpus.

Hello @Majestad , Dell specs says your system can support up to 675w of graphics, total of 3 x 225w, and your power supply met the requirements.  Add the second CPU and your setup should work fine.  Put the pair of RTX 3060 on slot 2 and slot 4.  Put the RTX 3070 on CPU2_slot 1.  Just be sure to get identical specs for your second CPU to avoid any issue.

Thanks. It has sense.

There are several problems you face.  First, with one CPU half the PCI-E slots in a T7910 are inoperative.  So you face a limited amount of real estate.  So you'd need to find a matching (same chip, same die lot) to enable the other PCI-E slots.   Second, even one cpu is very cramped.  I fit one 2080Ti in my unit, but I wouldn't want to try for a second.  I'm pretty sure that I couldn't even fit a second one.   If you do go for it, make sure you get founder's editions (or whatever you call the more slender versions that exhaust out the back).   Third, there are power limits.  Or you could use a PCI-E extension cable to a box with its own power supply.  There's a range of these but few hundred bucks gives you space and power.

I should have said "even one full size graphics card is very cramped". 

 I have a similar problem. I just got used T7910 and have 2 E2630's for 16 Cores and 32 threads.

I've got 96GB of RAM distributed evenly over both processors.
I have 2 GTX1050 TI installed under Zorin Linux, but can't get the one on second processor to show a screen,

Other than that I have no problems. 
I know NVIDIA and Linux drivers are dripping, smelling, viscous, solid human waste matter,  but that's what I got.