Is there a reason you don't want to just bike? Seems like the perfect distance for it, and it's free after the initial investment of a couple hundred dollars.
simply not a practical option for everyone. I only do it because I can bikeshare one way home. After biking a few miles in DC in the summer I need a shower and complete change of clothes.
Also, before they put the bike lanes in biking during rush hour felt stressful and dangerous.
I very much do want to bike, but my family (and some of my bike-commuting friends) seem to think I'll get hit by a truck and are very against the idea. :P
Depending where you live, there may be resources to help you get started. 3 miles is a perfect biking distance. Bike-commuting friends don't want you biking? Is there something particularly bad about your surrounding environment?
Depending on that 3 mile commute (hills, traffic, bike routes), it may also be faster.