TOPIC:

If you have 3 hosts a pool, can you install ISCSI-HA on two of them and use the XenServer HA controls on XenCenter?

If so what starts DRBD on a reboot?

This scenario is not recommended since there is no way of knowing which slave host will be promoted to master in an HA recovery when using xenserver HA. If the slave without the storage is promoted your pool will lose access to all disk images.

In response to your second question, iscsi-ha is responsible for starting DRBD.