Looks like entering the sum of 52+53 into box 53 (deleting 52) will fix the problem. I only had one T3. I would do the same on each one if I had more.
Someone at Turbo Tax actually stated the lines below in a previous message 5 hours ago. They are not on top of this issue. They could correct the content of the error message on their site but haven't. None of their support staff seem to know what's going on. They could be reading and distributing the info on this site (their own) to all support staff.
"To proceed with your filing:"
"• If you're unsure which period the gains relate to, you can use Box 53 for the amount shown in Box 21."
