This is only possible if you already have a motherboard that has several PCI-e x16 connectors. However, buying a dedicated motherboard is not good because the price of a motherboard with 7 PCI-e x16 connectors is over 400 euros. Also the processor must support a lot of PCI-e lines, e.g. i7 or i9 with 40 PCI-e lines (e.g. socket 2066 etc.). You know that the processor for that socket is extremely expensive. However, even then you will not achieve full speed on all graphics cards but only for two and a half speeds for the third (2 x 16 + 1 x 8 = 40 PCI lines).