HI sarkmo
Find answer below.
Can I utilize that single VM (as the VUM) and use it to upgrade all the ESXi Hosts?
ANS > Yes, One VUM server is enough to handle updates/upgrades on all residing esxi host in a same vcenter server. . So no Worry.
do I need to build 3 VUMs to update each Host?
ANS> No, No need to build 3 vms for your 3 host.
Thanks.