It does seem very overwhelming when you're tandem nursing and find out you're pregnant. All the what-ifs and wonderings, combined with the emotions surrounding the pregnancy and birth can be difficult to deal with at times.
I tandem nursed through my daughter's pregnancy and triandem nursed for 18 months afterwards until my oldest self-weaned on his fifth birthday. The bond my three children have is absolutely incredible.
I suggest, if you don't have it already, you get the book "Adventures in Tandem Nursing" by Hilary Flower. It has some info on tandem nursing through pregnancy and triandem nursing. (And my story is the first mother's story
How do you deal with tandem nursing during pregnancy? The best you can! Our fears and visions of what it's going to be like are often worse than what it actually is. Figure out what works best for you and your family. Try to take time out for yourself when you can, whether it's locking yourself in the bathroom for 10 minutes or having you SO take care of the kids while you take a walk or have a mama's night out every so often. Many moms find taking time for themselves makes the nursing time easier. Don't be afraid of setting limits if you need to. If you're not happy, your kids won't be either. Some coping mechanisms ... for when you're experiencing the "can't-I-have-my-body-back-for-five-minutes" feeling ... include limiting the duration of nursing sessions (by distraction, by playing games including counting from 10 to 1, yelling blastoff as you de-latch the nursling, and then zooming him through the air; etc.), limiting the frequency of nursing (by talking with the nursling and explaining that mama is tired and needs a rest before nursing, or that yes they can nurse, but after ____, etc.), practicing relaxation techniques (progressive relaxation, breathing techniques ... also great for preparing for childbirth) or taking a 'nursing vacation' (one mom used a light w/ a timer...when she was really "nursed-out" she'd announce to her kids that she'd be "open for nursing" only when the light was on. She had the option of turning the light on herself or waiting for the light to turn on via the timer, which she set so the light would be on for 30 minutes every 3 hours, as that was what she was comfortable with ... she would take her nurslings' needs into consideration and never refused when a nursling really needed to nurse, but it helped her to weed out those I'm-bored type of nursings and her kids thought it was a wonderful game. Her 4 year old even made her an "open/closed" sign to wear around her neck!)
Tandem nursing 3 is often called triandem nursing. It sounds overwhelming, but often our impressions of what it's going to be like are a lot worse than it will be. Some moms find assigning the older nurslings to one breast and the baby to the other works best. That way the baby isn't on anyone's "side" and the older two can usually cope with sharing. If that doesn't sound like it will work, assigning each of the older child a breast (so they don't fight amongst themselves) and having the baby share with both of them often works.
Introducing the concept early that breastmilk will be all that the baby can eat and that the baby will have first dibs on nursing seems to be important. Many moms have the fear that the older nurslings will see the baby as an intruder on their turf (your breasts), but illustrating that it's a NEED for the baby (role playing with a doll helps a lot!) and that the baby can't eat anything else usually takes care of that.
My boys "taught" their little sister to nurse. It made the older nurslings feel important and gave them an immediate roll in the baby's life. We encouraged them to tell Haley all about mamamilk and nursing and how wonderful it is. They really enjoyed doing so and were overjoyed when Haley would nurse and were her cheering section.
As far as either of your kids weaning ... many moms find that waiting and seeing is the least stressful way for everyone. Moms who "plan" for an older nursling to wean (or not) often find them stressed when they don't (or do). If a child is ready to wean, they'll wean. If not, they usually won't, or if they do, they might be interested in going back to the breast when the baby is born. Each child is different.
Yikes! This got long! HTH and if you have any more questions, please ask!