Most of the time I can get what I want out of SQL using the standard aggregate functions, but today I needed to find the latest timestamp from any one of three columns.

I achieved this using the GREATEST() function, it's not party of the SQL standard but is commonly available.

GREATEST(value [, ...])

LEAST(value [, ...])

The GREATEST and LEAST
functions select the largest or smallest value from a list of any
number of expressions.