With Update Manager in place that's the preferred method to patch hosts. However, in order to male sure that all hosts run the same build, you'll need to find out which HP components need to be updated in addition to the VMware patches. As an alternative - in case the 2 ESXi hosts run the HP CD-ROM edition without further updates - you may consider to patch the host interactively using the HP CD-ROM/ISO that was used for the other two hosts. A reboot of the host you are going to update is required in any case.
André