I would actually try calling at midnight on the day before (for ex. if your flight is at 7:30 am on a Sun. I would call at 12:01 am on Sat. rather than waiting to call at 7:30 am on Sat.) and I would definitely ask if there are 3 seats available (together if that's impt.) before proceeding.
I agree that the Air Tran CS people have been most helpful whenever I've called. But I have been given conflicting info. about
when you can call to upgrade. I know the policy is described on the website as "24 hours before" but once when I had called to ask some questions about redeeming vouchers the CS person told me I could call after midnight. I shared that info. with a friend who was flying a few weeks before me & she did just that with no questions asked. When it came time for our flights I stayed up to call at midnight & the CS person I got said I had to wait for 24 hrs.

DOH! I knew that my friend had just done it tho w/o any problems tho so I hung up & called back. The NEXT CS person let me do it.