Look at the code I came up with for my mqtt fan in this post:
It’s for an iFan03 but it has a similar structure to what you need.
My fan expects 0, 1, 2, 3 for your off low medium high but if you substitute your values in for mine it should work for you.
The only other thing you will need is to figure out how the insteon light reports it’s state.
Mine uses a json coded value called “FanSpeed” so you will need to change that to whatever your integration reports.
If you can’t get it working let me know and I’ll help you dig into it deeper.