I mean the ideal nice way of doing it would be with a bigger PSU, but I guess they aren't mahoosive cards so you should be ok power wise. Any way you wire it ultimately isn't going to make much difference as it's a single rail PSU, i'm no PSU pro and this is all a bit ghetto IMO.
The neatest way would probably be with 3 8-pin to dual 6-pin splitters and then just 3 cables from the PSU, which is basically the same as having pig-tail connectors which is fine for 300watt cards so I can't see a problem. Obviously if you only have to splitters use two PCIe cables on the third card.
Alternatively you could put one proper PCIe cable to each graphics card and then put the 4th PCIe on the master graphics card and molex adapter the remaining two 6-pins but that would be fugly cable wise.
JR