Agree, but no mother board will take more than 2 cards
You might want to look at one of the 4-Way SLI /Crossfire MotherBoards. Eg.
Thou they generally don't come cheap (unless used). And you may need a CPU with more than 16 PCIe lanes (also not the cheapest). Some of the older X99 CPUs have 28 or 40 lanes for example. Plus, powering the GPUs directly from the PSU... And a Case with enough slots etc... And more cooling...
And after all that, is it still economic?