There is also a two year NROTC program. You need to apply via the unit at your school during your sophomore year. If accepted you will attend a 6 (?) week program in Newport, RI (?) to catch you back up to everyone. This is probably much more difficult to get into than trying to do it in three years. Speak with the officers at the unit you plan to participate in. They can shed far more light on the paths available to you.
I agree with Grunt that you won't have to make up labs, but I expect there is a possibility you will need to double up on the academics. The first year academics aren't too difficult though, so even if it is required I've no doubt you can do it fairly easily.