Processor: Intel i9-9900K
Memory: G.SKILL Trident Z DDR4 64GB 3600
Graphics Card #1: Asus ROG MATRIX RTX 2080 TI
Graphics Card #2: Asus ROG MATRIX RTX 2080 TI
Case: Asus ROG Helios
Power Supply: Asus ROG Thor 1200W
My concern with the 115X boards was always the low number of channels the CPU has... Whenever going with an SLI setup, I used the bigger boards - simply put, the ones with quad channel memory (those CPUs have more channels). So, Intel LGA 2011, 2011-v3, and LGA 2066: choose EATX, usually the PCIE slots are spaced pretty good for an SLI setup - however, the 3-slot graphics cards will be still pushing the limits...
Now, price-wise it worth considering the AMD boards too: also EATX, but only the X399 chipset ones - those are for the Threadripper CPUs. At this moment, I would wait though with these AMD boards... I have a feeling (from reading here and there) that AMD is coming out with the new Threadrippers soon...
The X570 AM4 platform would have enough PCIE lanes, but the dual channel memory setups are bothering me... That's why I was suggesting the X399 or it's successor...
Overall, your problem is with the 3-slot wide cards... not too many boards are designed to allow enough breathing room between them. As said before, placing water blocks over them is the easiest way to address the issue...*