Sorry, I have no idea why there was a 20 min outage, I never experienced anything like this myself. VMware made a couple of improvements regarding snapshot deletion, so there should not be an outage like this with the latest versions of ESXi anymore!?
What you could do - if all three snapshots show up in the snapshot manager - is to delete the oldest 2 snapshots one-by-one (starting with the one closest to the base disk). Since this does not require the VM to switch to another .vmdk for write access, this should not cause any outagel. Deleting two of the snapshots would at least free up a lot of disk space and you could try to schedule deleting the last/current snapshot in a maintenance window.
André