I implemented something similar for this need just yesterday. Our situation was similar. 3 machines total, 1 was the 'head' and 2 were simply serving up their data via iSCSI target.
I used openfiler as the head. They do not officially support mounting iSCSI and re-serving them as native NFS/CIFS however it was trivial to log in via ssh, mount them, and proceed normally within the openfiler GUI to add them to the volume. If you use a versatile file system (XFS), combining them all and expanding over time should be pretty easy.
Under the hood, this is nothing more than openiscsi+lvm+samba/nfs but it tends to work pretty well.
I did, however, have some issues since a) its not supported and b) its a bit kludgy. I was able to get everything to start properly on reboot, including mounting the downstream iSCSI targets and having it serve properly from a reboot, however I expect complications should an individual SAN become unavailable to the rest of the kit.