A few of things to check....
Please double check the version of code running on the RAP-3s. The symptom you describe is typical of SW mismatch. I'm not sure if 3.3.0.0 has been posted to the cloud based SW upgrade server so the RAP-3 will never find matching SW and will never join the group.
Also double check the port on the RAP-3 you are plugging into. Eth0 is the top port not the bottom port. If the uplink is not connected it will not be able to join the group.
How many IAP-9x do you already have and how many IAP, total, are there in this group. The IAP-9x is limited to 16 per group and it will hold the entire group down to 16 max. No additional IAPs will be able to join if the limit is reached.
Check the regulatory domain of each IAP. Only IAPs of the same regulatory domain can join a group. Ex. -US IAPs can only join a group of other -US IAPs.