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.