Google Sheets: SUM of the LAST value - apply across multiple columns

Use Case: I have multiple financial portfolios that cannot be connected to via API. I want to track my total balance across portfolios. I do not want to tally every portfolio every day - instead, I want to update individual portfolios occasionally, and have a running "up-to-date total"

This would be easy if Google Sheets had a REDUCE function. Here's what I've tried:

=ArrayFormula(SUM(INDEX(B3:D,COUNTA(B3:D))))

I've also tried removing SUM and trying to just print the last values of each column using ArrayFormula.I have also tried using =ArrayFormula(LOOKUP(2,1/(B3:D>0),B3:D)) as an alternative to get the last value of each column.

1. How can I calculate the sum of last value across multiple columns?
2. Why does GSheets not have a REDUCE function?