I'm using an Asus Prime Z270-A (and a Intel G4400) with 6x GTX 1070 and 1x GTX 1060. The mobo has by default 7 pci-e slots and I think you could use 3 gpu's without any risers. https://www.asus.com/be-nl/Motherboards/PRIME-Z270-A/

As a psu, I'm using a Corsair AX1500i.