If you can find a remote with an LCD display that can be customized with images, it might help. That way, she can make choices, based on images, rather than text.

Thinking of a 3 y.o. is hard to do and sometimes, the abilities are unexpected- I was programming a Harmony 1000 and the homeowner's 3 year old daughter wanted to watch a DVD- she sat next to me on the sofa, looked at the remote, pressed the DVD icon, waited for the transport buttons to appear before pressing Play, then sat back to watch the movie.

I had said absolutely nothing to her about how the system could be controlled.

Try using an iPad-based control program, with icons- I think it might be the way to go.