Go for an HP board with 3 PCIEX1 and 1 PCIEX16 which can support up to 3 or 4 (not sure about 4 graphic cards). Cpu an G3240 or even G1840 which is same generation. Both will be 80 Euro new ,and much cheaper used ones.
However keep the board open as I don't think any tower will fit 3 GPU-s into such board, as you have to use risers. I have tried one of these at my work, the HP Motherboard Pegatron ipisb-ch2 v1.01 and it worked good with nicehash and 3 RX 480 , of course I changed the PSU to Thermaltake TR700 Gold.
You should look for a gen4 motherboard though, as the one I used was a lga1155 old one, yet still running.