Just be aware that the UPH was designed to be a "Length of Stay" pass. Going by the letter of the law, for a 3 night stay, the only UPH that the resort would be able to sell you would be 4 days / 3 nights, beginning the day you arrive and expiring at midnight the day you check-out.
Unofficially, WDW resorts will allow you to drop EITHER the arrival day or departure day. Occasionally there are reports of CMs who are willing to drop BOTH the arrival and departure days, but quite honestly you may or may not be so lucky.
Best case scenario--the CM would let you drop the arrival and departure. You would pay for a 2-day UPH and have full park access on the middle two days of your stay. Or, you could be restricted to just one dropped day, in which case you'd pay for 3 days and have to choose which day to drop.
Unless you plan to make use of the waterparks, PI or DQ, paying for the UPH is really not very economical. You might consider buying a 4-day park hopper, and just hanging on to the unused days for a future trip.