I don't have a definitive answer for you, but let me think out loud here.
First of all, the backup you have for the iPad 3 - is it backed up to disk via iTunes? If so, I don't believe there's any way to combine it with anything else; the act of restoring it to the iPad will wipe any existing data on the iPad.
What I'm thinking is, you may be able to combine the two sets of data by using an iCloud backup in conjunction with an iTunes backup. Set up your iPad 5 to back up to iCloud. Once you've got a complete backup to the cloud, do a restore from your iPad 3 disk backup. If I'm not mistaken, once the restore is complete, the iPad will then go on to restore anything that's backed up on the cloud that's not already on the iPad.
BTW, probably wouldn't hurt to do an iTunes backup of the iPad 5 first just to make sure you don't lose anything in case I'm completely wrong about all this, but I think it'll work. Hopefully some of our more expert contributors can chime in and confirm or deny my theory.