What do you guys recommend here?

I have 4 servers each with 3 HDs, I used to have them setup as a cluster VSAN but after I upgraded to ESXi 6 they're no longer supported. So now i have 14TB across 4 servers that I would love to pool into one giant drive. I can't use Storage Spaces Direct because that requires 5 drives per node.

Does anyone know of any other software that will let you pool drives across servers and present them as a single LUN?