we have our app server on 8.9.04.200 version and RSCD agent on few servers are on 8.7.04 (which we are in process to upgrade) , the communication is working for every operation except Windows patching. So ideally your scenario should work.
Patch 3 of what version and service pack, 8.9.04 ?
Agents on higher version than appserver are usually not supported.
If we upgrade our agents to patch 3 of the rscd agent , will it be able to connect to application of BSA.
i'm assuming you mean 8.9.04 patch3 ? if so, the answer is that we don't support older appservers w/ newer agents.
also - the agent never initiates a connection to the appserver. it's always appserver -> agent.
why do you want to upgrade the agents and not the appserver ?