I figured out the formula to give me exactly what I need in Excel:
=IF(LOOKUP(2,1/(A1:D1<>""),A1:D1),EOMONTH(LOOKUP(2,1/(A1:D1<>""),A1:D1),3))
This formula does not translate to smartsheet. I need a formula that finds the last non blank in the row and auto fills a due date of 3 months later, last day of the month.