I don't like to be condescending but unfortunately in this case I must...

If you are unable to modify the AHK script from 2 to 3 windows then are you sure multiboxing is right for you?

this is one of the simpler tasks you will face, the code is already written for you and it is simply adding an extra line for each hotkey.

Reading the enclosed help files is more than adequate to give a complete novice at scripting the basics for getting going using AHK.
This forum has numerous examples of ahk scripts and at least one specific to tri-boxing (i posted one of them myself)

If, using these 2 resources, you can't modify the script then perhaps using one of the tools created specifically for this purpose (which can be found in the software forum) is more suitable for you.

Finally, this community is happy to help, generally through provision of information and learned experience. But it does require effort from those participating in it. There is no spoon feeding here.