I'd like the field to be a pop-up that shows the last 2 or 3 values entered as choices.
The last 2 or 3 values entered by that user or by any user? In other words, what rules would you need to apply to identify which values need to appear in the value list?
Sometimes it's easier to pop up a small dialog type window with a portal that lists the values you want to show as this method provides more sort and filter options to control what gets listed in the portal. Such a portal can have the fields that fill the portal row set up as a button so that clicking a portal row selects and enters a value from that portal record.