3 x PC 5548
MD3600i which requires 2 subnets (plus few other iSCSI arrays)
Ofcourse I could do 1 switch per each subnet (with one switch spare in case of failure)
But is it possible to serve both subnets over 3 switch stack
It is iSCSI only dedicated switching (no other connections to it)
Yes you can use VLANs for the separate subnets. Here are some good deployments guides that may help in setting up the MD3600i.