systemsaddict I might have a few strategies that will help. I have 3 children (4.9, 3.1 and 17 months). The eldest has SN and needs assistance with dressing, and has behavioural problems as a result of her SN.
-I find that giving too much time is a recipe for disaster - as you have found, it gives them time to undo what I have done. So, I leave getting dressed and hair until just before leaving. Literally, no discussion, just single words to keep focus. So, "Pants". "Socks", "trousers" "push", etc. DD1 can get wound up if I use too many words - it is just too much to focus on.
-I don't know what kind of car seats you have. Mine are still in 9mth-4 year seats. I find that the key is not to fight them. Make the straps as loose as possible, click the straps together, then hold the tail of the strap ready to tighten, distract, push gently on the tummy and pull the straps QUICK. If I fight DD1, she 'stair-rods' and it is impossible to get her to bend in the middle. A quick distraction and a gentle but firm push on the tummy makes them almost reflexively bend, and before they realise they are being strapped in, the straps are tightened. Also, try not to have a coat on them because it is much easier to do the straps up without.
-I don't know if you have a driveway, but I do. I always take them one at a time to the car, so that I am not trying to deal with a circus act. I take the youngest, strap her in, go back for the middle one, then the eldest.
-I find that "1, 2, 3 magic" is very effective. A quick explanation of what I want them to do, then count to 3, then consequence. No discussion. It took a little while for them to realise that I will ALWAYS follow through (trick is to make sure that you choose the consequence carefully - even a very simple one such as "if you don't do x, I will have to do it for you..." seems to work). Again, I find that too much discussion adds fuel to DD1's furnace.
Keep going, it is hard work 