Adding to the above,do find the below on how the traffic traverse between AP3805 and ExtremeCloud.
I believe the below link on how to add and manage AP with Extreme Cloud,would help you.
Thank you for raising this post.
AP 3865e platform support only 3×3:3,3 spacial streams at time.
The antennae has 2dbm gain.
Please let me know your requirement for installing 6 antennae.
Proper sight survey has to be made before deciding ...