+x
If you are planning on using SkyDemon on three devices, like you have stated above, then yes I believe you do need a third mobile licence.
If you simply want to use your phone as a backup in case of hardware failure, then putting up with an error message on the failed device should not really be an issue, as I would assume once back on the ground you would re-log back in on the failed device and the working iPad and this would then log-out the backup mobile.
Just out of curiosity, what are you using two iPads for simultaneously showing SkyDemon in the cockpit?