Does that mean that when you give the modelled lesson you read from the lowest level? Or the highest? Or would you need to do it in 3 different groups? Which level do you think is best to model from if you have those three different levelled groups?

Ideally, it would be three different groups if you have sufficient resources at the school. You can model your lessons from the lowest to the highest level. However, if there are time constraints, you may get the best results by focusing work on the middle level.

It takes good classroom management to guide them all through different texts and levels at the same time even if the instructions and strategies are the same.