idk how to do it, i defeat two of the commanders and then the third one has too many armys for me to take him out, i looked up walkthroughs and the AI's went in different directions then they do for me.

The key is to spread faster than they do and to focus on each commander when they are within reach (put everything you have against them when they are bordering your, but don't put just what is needed to take while you are not adjacent to them).