I'm answering your question assuming you are moving to Birmingham.
After several years of a high priced market, prices are down in Birmingham. It's a great time for buyers. Interest rates are going down and will continue to do so over the next couple of months. Birmingham is a growing city, so as long as you are smart in your purchase, you should be able to sell in three years.
As far as turning your house into a Section 8 rental when you move in three years: I know investors who buy only with the intentions of renting via Section 8. They do very well, they know they will get that payment every month, but they also know there is a risk of property damage. If you aren't prepared to keep your property up, and don't want the probability of having to remodel down the road (should you want to move back to the property), then renting the property may not be financially feasible for you.
There are a lot nice apartment communities in our area should you choose to rent instead of buying your own home. Some areas of town are rather expensive though, so buying may make more sense.
Good luck to you and your family!