Okay, I understood the question to be how your BODY can cope with three close pregnancies. There are things you can do now.
Start drinking red raspberry leaf tea, a few times a day. Start now. Drink it through your pregnancy and post partum as well. Some people like it better with honey in it.
Do Kegels. NOW. Ten sets of ten. Every day.
Get out and exercise. You have two little ones? Put them in a stroller and walk. Briskly. Every day. EVERY DAY. For a minimum of a half hour. When that gets easy, find some slight hills, or go longer, or go a little farther.
Get a book called "Expecting Fitness" by Birgitta Gallow. Follow the routine.
Eat REALLY, REALLY well. Go to blueribbonbaby.org, follow the diet. It's easy to follow, and your body needs the healthy food and the proteins to not only heal from the last pregnancy, but to grow this one well and protect itself, as well.
So, there's a higher incedence of hemorrage after pregnancies that are close together, because your body doesn't have time to recover. (I know this b/c I had two pregnancies close together and hemorraged ten days post partum...) This is why all of the things I've listed are so important. You need to help it heal, support the pregnancy, AND protect itself.
The rrl tea will help to tone your uterus, and help a little to protect it against hemmorage...it does this by toning the muscle and helping it have "memory" as to how to clamp down effectively. The kegels will help to tone your pelvic floor and protect you against prolapse. The exercise will feed your body with oxygen, will help to strengthen the muscles you need in labor and birth, as will the book. They will also make for a shorter more efficient labor than you might otherwise have, AND an more healthy, toned baby, who is more able to undergo stress in a healthy way. The diet gives your body the nutrients it needs and the building blocks it needs for it's multiple jobs.
Rest, every day.
Yoga is really great too, as "self" time and toning and breath exercises.
Good luck.