Whew! Parenting is definitely a struggle sometimes.
Fact: Some kids are definitely more challenging than others; some are super-easy in comparison.
My older daughter has been a real challenge for us emotionally and physically. We noticed her extreme moods in the first year, and bought "The Fussy Baby" by Dr. Sears. I wasn't real happy to read what it had to say, and I was really tired, so the book kind of disappeared. In recollection though, I can say that this book was pretty spot-on with my daughter's moods and behaviors.
A few tips:
1.) Please don't spank or hit. I know how difficult little ones are, and how frustrating they can be at times (I was a stay-at home mom too). But, spanking almost always comes back to bite us in the rear later.
2.) If your son wants to wear pj's all day, go with it. Even if you're going somewhere...who cares?? No one, but you maybe. It isn't worth all of the stress of the struggle.
3.) Save your struggles for the really big issues. Otherwise, don't make an issue out of it. When your son is kicking and hitting, ask him if he wants a hug. Assure your son that you love him, but hitting & kicking is not appropriate. It is NOT allowed. Give him other ideas on how to express his frustration. Talk about what's bothering him, give hugs, etc. It is a bit difficult to get this ball rolling, but it does work! It just takes some extra time, calm demeanor (on your part ~ even though you feel a lot of emotions inside), and persistence.
4.) Make sure that your son gets special time to spend with you alone, as well as special time to spend with daddy alone too. It can be tough to put into your schedule with 3 little ones ~ but your son needs some extra attention. That's why his behavior is going a little haywire right now. While it's negative attention, it is attention that he's getting from you. So, make sure that your son gets lots of positive attention!
5.) Set a timer ~ When it goes off, then it's lunchtime; playtime; end of playtime; and so on. Kids do really well with the timer! As they get older, you can tell them, "Five more minutes, and then.." Let it go from there. Don't continue to repeat yourself. If he continues to press you about how long it will be, refer him to look at the timer. He may complain that it's SO LONG, ask him to draw a picture or read x book to you.
6.) Then comes the tv... Personally, I have always been a stickler for my girls not watching too much tv, and sticking to educational shows. Kids get hooked so fast ~ and it's really not the stimulation that they're looking for. IMO, try to limit their time to less than an hour a day. Otherwise, keep the tv off or left onto a music tv station to listen to. Use that hour wisely ~ during a time when you really need a break (snuggling with your baby, eating in peace, etc.); otherwise, stick to 15-20 minute intervals here & there.
I had a great book (which I've held onto) that teaches fun songs to sing and play with little ones. "Songs and Games for Toddlers" by Carol Gnojewski I still sing a couple of the songs in this book to my girls
every night as they're going to sleep. (They're now ages 9 and 11!

)
I am not trying to sound as though I'm a perfect mom, so I hope that it hasn't come across that way. These are just some things that I have learned over the past 11 years. There have been times where my older daughter has scared the bejeezers out of me! She is now almost as big as I am, and she likes to remind me of that every now and then.

Not fun! Then, despite my fears, I use those same techniques & tell her that I love her very much, but I don't like that thing/s that she said & did. (Always done after the dust has settled though) First thing is the hug.
I hope that this helps you ~ gentle hugs sent your way. You have got a lot on your plate with 3 little ones under age 5 at home to care for.