You've had some great suggestions for once he has left. May I just add something for the mean time?
It sounds like your 3-year-old is getting worked up over the anticipation. I would stop talking about it around him. I think that, for him, the thought of it happening is the worst part right now. He doesn't have the logic or reasoning to understand all of the things you will do to keep in touch. Reminding him about it, even to make plans for keeping in touch, is just working him up.
If he brings it up, reassure him that it will be okay, and that you'll be able to talk to daddy every day (I agree that Skype or something similar will be very good). But don't let him dwell on it. When it's time to say goodbye, help him through it, and know that he'll probably cry a lot. Remind him "we'll see daddy soon" and help him give hugs and wave bye-bye. Make sure your husband calls as soon as possible once he arrives.
My husband used to drive cross country. It was hard on all the kids, but especially my youngest (5 at the time). He cried bitterly every time daddy left - he was gone for 4 weeks then home for one. During the week home, we wouldn't talk about when he left again, we just enjoyed the time we had together. We talked every day on the phone, and the kids were free to call him whenever they needed to. I think it was harder on them that he kept coming home and leaving again.
I would also make sure that you have support for you while he's gone - being pregnant and almost a single mom can be really draining. Do you have people who can take the kids for a few hours and give you a rest once in a while?