Have you ever been short-distance? I don't think you should be thinking about marriage unless you've been in the same place for an extended period of time.
If you have had an extended period of time in the same town, I don't think you should wait over three years to marry. 1) He could die or be seriously injured 2) Depending on your age and your mutual fertility, there may not be any kids if you wait too long 3) How much of the three years is he going to be deployed? It can't be more than one year, two at the outside. You might not even be able to conceive within that time.
I have known a number of military families in our immediate circle. While I have one military cousin whose marriages have been a disaster, that's not because of the military, it's because he does everything way too fast. My other military connections seem to do OK, even with a biggish family. If you are up for doing some solo parenting and moving a lot for a couple years, you'll be OK. Three years is nothing in the life of a family, but it's eternity as an engagement.
Maybe he's a good guy, but this sounds very excuse-y.