As I know, connecting an iscsi LUN to multiple targets and then presenting them to several ESXi hosts cause problems. Because each host connect using own target to storage LUN and it conflicts with the IO request.
Based on my experiences a single target is better because all ESXi hosts are active and only one target manage IO requests.
If your storage device support multipath input-output (MPIO), you can achieve greater throughput by using multiple targets.
Pay attention you must present all target to all server. (each server must access to all target)