I have a 3 year old and 13 month old so I can understand the juggling of 2 and getting them into car seats. Some things that I have tried is the art of distraction. I have realized that my 3 year old loves to be my 'helper'. I usually give her a job like carrying her own bag out from the grocery store. Makes her feel big. Also, I give her a goal to shoot for when she gets into her car seat such as a cookie (yeah- I know bribery), book to look at or toy that I hold onto until she is completely in the car seat and at least waiting for me.
Oh - I also play 'choo choo' with her where she hold onto my purse, coat, etc. and we say choo choo into the place and it becomes a game. (sorry this probably goes more with the running into the street)
It does get better. She is definitely testing you which happened with me when I had the second child. I made special trips to the grocery store with her at night sometimes to spend time with just her. It helped. I still make 'dates' with just her and is a great reward and she loves to have mommy all to herself again.