Just an FYI - I gave up on TurboTax figuring out that the problem lies with TurboTax. They "say" that Box 52 + Box 53 = Box 21 but they aren't actually coding that in their error checking.
I gave up on TurboTax - downloaded another program - re-entered all my info and Voila! Netfile success this morning. No problem with my T3's at all.