@Richard_Otter 

The addition of a controller offers central management for the entire WiFi network as one unit, rather than 3x individual devices.     

Personally the main advantages of this for me are as below

1. I can see all the clients connected in one portal, rather than having to jump onto 3x separate devices
2. I can manage the APs from the same portal, they will also pull config from the controller and therefore should avoid interfering with each other etc. 
3. Reporting is better and the interface more pleasing
4. There is certain features you can get without a controller (Mesh, Roaming, Seamless, AI)
5. The EAP range are primarily designed to be the controller managed
6. Last but not least, seamless roaming...  The controller will talk to all 3x APs and move devices between them without disconnecting as required.  It will even proactively move users if it feels a roam from A to B will offer a better signal/speed.

My honest opinion... if you are going to the extent of running 3x APs, then the controller is likely a good idea for you.   The OC200 for example is a fairly inexpensive device and will just offer you a much-improved experience for setting up and managing the WiFi, especially if you are new to WiFi Networking.    

You will also find most people on the forums here are running the controller, its generally the most common choice.  However it is your decision!   Feel free to ask any other questions you might have

Philbert