My mortgage was sold in 2018. On the new 1098: Box 2 is blank; Box 3 still has the original mortgage origination date and NOT the date in 2018.

Having the original date of the mortgage origination is correct.  You did not get a new mortgage, you are only paying a different company. 

The terms of your mortgage stayed the same as when you originally borrowed the money. 

Since you said that the box 2 is blank on the new mortgage company 1098, you may need to enter the outstanding mortgage balance shown on the previous mortgage company 1098 to proceed. 

[edited 4/6/19 | 8:52 am PST]

edited