Yes, you certainly can use same ACO in multiple instance.
Are these instance have unique hostnames for each ?
- AgentConfigObject="wac_web1,iiswebsite1.com"
- AgentConfigObject="wac_web2,iiswebsite2.com"
- AgentConfigObject="wac_web3,iiswebsite3.com"
Because if you dont' tracing the request on the policy server would be a problem while troubleshooting as you wouldn't know from which agent the request is coming from a quick look at it ..(you can always trace it based on transacationid though but it would need more work)
Other than that, for managebility purpose I don't think its a good idea to have the same ACO used by mutliple instance in PROD specially... for eg. if you had to enable tracing that would then apply to all the agents...