I did one like that a few years back, but I went from 7.25 to 6.0, and only kept the house for 2 more years. I had to pay a point to get the 6.0 as well.
You'll want to do the math. You need to get an exact dollar amount for the cost of the refi (points, fees, etc) and determine how much per month you'll save. Add up the monthly saving over the three remaining years (or make a conservative estimate of the shortest time you'll have before selling), and see what your savings are.
I personally wouldn't count on mortgage rates going any lower than they are now, but you should also shop around to find the lowest rate with the lowest points and fees. You might be able to get a better deal at one place if you have a pre-approval from somewhere else in hand that you can wave under their nose and ask them to beat.
I'm considering aiming for 5%... I'm at 6.25% right now on a fairly hefty mortgage. I might keep the house as a rental or sell it in 2.5 years, but I won't know until I move. That means I'll have to be careful about eating refi points and fees.