I agree w/ Amanda - change the title of your question to be "question for parents of several toddlers" or something similar.
I have only one kiddo, but I was a nanny for triplets starting when they were 8 months, and they had a surprise sibling join the group when they were 23 months. Oh my goodness, that singleton was more work than the 3 kiddos put together!
First of all, give yourself some credit. You are doing an amazing job - you read, sing, dance, color, cook, help and mother all day. It's exhausting. Here are some ideas:
If your middle child won't stay in the room where you are dancing, close the door or put up a gate.
If you need to do hair, put all three in booster seats, and let the other two entertain themselves with board books or crayons or a snack. Everyone's out of trouble and hopefully happy. If not happy, then safe.
When singing, ask each child what song he/she wants to hear. Then pick an order and say, "We'll sing Wheels on the Bus, then Twinkle Twinkle, then Old MacDonalds Farm. Ready? Wheels, then Twinkle, then Farm." and start singing. If someone doesn't like it, it's too bad. Part of being a sibling is learning to share - not only toys, but time. Continue singing over shouting and you'll eventually get to the desired song. The only exception I made to this rule is if one child melted down into an all-out tantrum. Then that child got carried to their bedroom, reminded that they were free to scream and cry as much as they needed, as long as the door was closed and I didn't have to listen to it.
Books: refer to tip listed above.
Teach this phrase: "My turn next". Sometimes MAMA gets to say "My turn next!" and then take your turn, singing a song that YOU like. If there's a struggle over who gets to go first, remind one or two children to say, "My turn next." or "My turn after that!"
I bathed all four kids at once. It was tight, but it worked. The first one out of the tub was the most reliable child (toweled off and stuffed into jammies). Then the second most reliable, and so on. The least reliable (and mostly likely to make trouble) got out last.
For teaching them how to dress, again install the gate in the bedroom door - keeping all of you in the bedroom. If one or even two kids aren't interested in how to put on pants, they are probably paying attention at some level.
Most of all: if your middle kiddo has PDD (I'm assuming you are referring to Pervasive Developmental Delay - if I'm wrong, forgive me), it's not your fault. It just happens. You can't prevent it - there wasn't anything you should have or shouldn't have eaten during your pregnancy that would change that. You couldn't have prevented it if you held him more or less as a newborn, or if you breastfed vs giving him formula, or vice versa. If you've had him evaluated, move on to the next step - OT? Speech? Inclusive preschool starting on his third birthday? My child has global delays and I know what you mean about leaving him quietly alone to play with that same toy that isn't teaching him anything, but at least he's absorbed and not stuck to my leg.
Keep reading to your children. Recite a story in the car. Have a conversation about your day - even if you are asking and answering your own questions. "Kee, did you see the big dog running down the sidewalk? (pause) Do you remember what color it was? (pause) "I think it was black. But it went by so fast I didn't really get a good look. Maybe it was brown." (pause) "If you were a dog, what color would you want to be?" (pause) "If I was a was a dog, I would like to be BLUE!" etc.
Also - don't hesitate to put on a short pre-screened video two or three times per day. Give yourself those half-hour breaks. It will make you more sane and a better mommy. Personally, I like Caillou (the newer videos w/o the "puppets" in between segments). They're calm and age-4 Caillou is very well-behaved, thoughtful, and not whiny. Again, don't bother with the earlier "puppets" versions - he's more like a real kid in those videos: whiny and downright irritating. PM me if you'd like the titles to those we watch. Or should I say: HE watches while i do something else!!
I hope at least one of these ideas is helpful. I'll be interested in reading what other seasoned parents have to say!
Edit to add: kiddo also likes sign-language videos, particularly "Signing Time". Educational, it's helped us with some of the communcation issues, and when he's misbehaving or screeching at me, I can sign "Stop." or "Later" or "Wait" and it's more efficient for us than when I raise my voice.