How to assign heros to legion?

Open your legion list and select any legion to then be able to drag and place heroes in the 3 slots on the left. However, you need to unlock the slots for the heroes to go in through research if you have not already done so.

In the research menu each of the legions that you have available will have an individual research tree. The first item will unlock the first hero slot. Then half way down is the seond clot research and the final research item unlocks the 3rd slot.