I don't think there is a simple point and click answer for you. You have 3 machines out of 5 that are at capacity. Your choices seem limited to either getting larger machines, or developing a capacity management system where users are shunted off to other machines; either due to server usage level or a random(ish) assignment when logging in or tasks are being performed.
You also need to be sure that you don't simply have too much load on your DBMS (Oracle, whatever).
If I were faced with this problem, I'd sit down with some of my peers, and discuss the work that is actually being done, the hardware capacity, and the network topology. Look for ways to rearrange things to lighten the load. If you can't rearrange software, hardware, network, or database, then larger servers are in your future.
I did a google on "linux load management" and got a number of good hits. Give that at try and pay special attention to load balancing software. Here is one *random* hit that looks relevant to your problem: