Is installing ECC Master Agent on ESX 3.5 supported by VMware. I mean, is this software officially certified or something?
ECC 6.0 does provide a VMWare agent to monitor ESX 3.x and VC2.x environment.
This agent must be installed on a Windows host, it'll discover the VMWare environment over IP.
It's not required (and supported) to install an ECC Master Agent on an ESX Server.
That said, prior to ver 6, you can install the ecc agent in the console session, and it's not terribly difficult. It is not supported, and further, is not really a good idea to begin with.