Easiest way to achieve this would be to "trick" OBIEE into thinking columns are different. Simply add a space at the end of your column formula on your report on dashboard page 3. For dashboard prompts to work with is prompted filtering, the column formulas in the dashboard prompt definition and the is prompted filter column formula must match.
Brilliant. Your answer works. I love trickin' obiee out. in fact, that would make a nice, nerdy t-shirt.