Sounds to me like your kids may all be reacting to your pregnancy, or maybe to your natural exhaustion. (Pregnant with three kids would exhaust anyone!) There's no easy answer to this, because just at a time when you naturally have lower energy, all your children need you more.
I would suggest focusing on preventive maintenance, combined with targeted support for the areas where each child is having issues. By preventive maintenance, I mean daily time alone with each child, daily roughhousing that inspires laughing for each child to help them work through pent-up emotions, and "scheduled meltdowns" as necessary to help them cry through any deeper issues. I think this preventive maintenance program will probably clear up your son's tummy aches and night waking. Here's a link with a preventive maintenance program for you:
Is this a lot of work? Yes. But it will meet your kids' needs. At this moment, each of them is showing you, in different ways, that they have some needs that aren't being met.
By targeted support, I mean focusing, one at a time, on the areas that are problems for you. You might want to begin with your 15 month old, who needs your help to learn to fall asleep without you rocking and cuddling her. This should be a gradual process, so she doesn't develop other problem behaviors by repressing her dependency needs. Here's a link on how to do this:
In the course of this process with your 15 month old, she will do some crying, in your arms, as you tell her that she'll be sleeping by herself. That's a good thing -- as long as you are there to help her feel safe. That crying is what allows her to clean out those feelings that are keeping her from relaxing into sleep.
As far as her screaming goes, this is very common in babies this age. This stage is hard on parents but reassure yourself that it doesn't mean she is becoming a willful brat. It means she has big feelings and no confidence in her ability to make herself understood. This stage will pass fairly quickly as long as you don't over-react, but simply stay calm and say "Oh, you want me to pick you up? Of course, come here....You can say 'UP!'" Of course, right now she can't say up, even if she understands the word. It is just a lot for her to put together, especially when she's feeling emotionally overwhelmed, as she often is. But this screaming is a temporary stage, that vanishes as kids acquire words. Don't over-react to it.
Finally, there's your four year old, who is clearly having a very hard time right now. You call this general toddler behavior, but a four year old is a preschooler, not a toddler. Yes, four year olds are still very young and they do things impulsively. But the description you gave is of a child who is really hurting. Sneaking food is usually a sign that kids feel there is not enough to go around emotionally. Waking up in the middle of the night begging to watch TV is a sign that she is feeling emotionally upset and wants to use the screen to pacify herself. The daily special time and roughhousing with her, as well as scheduled meltdowns and lots of snuggling, will go a long way to heal what's wrong for your four year old. But if you faithfully follow the recommendations in the preventive maintenance post and she keeps acting like this, I urge you to get some parenting coaching or other support before your baby comes. That will be another stressor on all your kids,obviously. You want these challenges resolved before your children have to handle a new baby.
I want to close by urging you to do whatever you need to do to support yourself as a mother. You have taken on a huge mothering load. You already have one baby and two other kids, and you're about to have another baby. Your children depend on you as the center of their universe, the person who helps them stay centered and regulated. Give yourself whatever support you need to, to stay relaxed and happy -- for yourself, for the growing baby inside you who feels your stress hormones, and for your three children who still desperately need you. Blessings to your family.