Q:

Build Pool/Agent Configuration in TFS 2015 for multiple collections

Currently, I am using TFS 2015 update 3.  we have total 4 collections.  
I have configured 2 Agent pools and each pool I had configured couple of Agents in Collection 1.(Up and running in pools/Agents)
Here is my question: Is it possible to use Same Agent pools/Agents in Collection 2,3,4? 
Please let me know the possible ways? 
Do i need to download agents and configure for remaining collections? 

A:

Yes.  Agent pools exist at the server level, not the team project collection level.  Agent pools are available to all team project collections.  
Within each team project that needs access to the agent pools, create agent queues for them that map to the appropriate pool. 
Please refer to the documentation, which explains this.