First, I'm not quite sure whether I completely understand your current setup.
But keep in mind, distance is not the only criteria for connectivity. In a mesh setup, AFAIK the APs would connect in between one another using the 5GHz band only, which can be impacted easily by obstacles like walls, metal etc.
There is little you can configure on ION at this point other than positioning the APs differently or connecting more APs to wired connections.
Hope this helps...
@MAikel Multi-hop over the air is not a recommended solution and you will get a 50% reduction in bandwidth with multi-hop. It sounds like AP3 needs to be moved further away from AP1 so that over the air signal is not received. If you are still running into issues please reach out to support to help troubleshoot your issues.