they do sound like normal kiddos... but rest assured, that sounds like normal frustration.
have you tried time outs? I find that yelling usually doesnt work. The thing I find it does is 1) lets the kids know that they are getting attention for bad behavior and 2) undermines authority because basically if i were to yell I'd be showing the kids that they can get a rise out of me and control *my* behavior (after a while it gets predictable and they try to push buttons, though the younger one may not have that concept down yet). worst of all, I've found that when I yell I wind up feeling worse for it (more stressed, guilty for losting my temper, more out of control in the situation, etc).
i dont like ALL her practices, but i really like the super-nanny's time-out-chair idea... Its helped us a lot with our dd who has autism and a lot of behavior issues. The idea is that you choose a chair (or a small rug even) as the "naughty chair" (you can call it the time-out chair or whatever else suits your fancy). Here's how it works, according to super nanny

~child is clearly told the rules.
~if child breaks the rules they are given one stern warning, and is told that if they repeat the behavior they will go into the naughty chair.
~if they repeat the behavior again they go straight to the naughty chair. they must stay there for 1 minute per year old that they are (so the 18 month old would stay for 1 1/2 minutes. the 3 year old for 3 minutes).
~if they get up before the time is up, put them back on the naughty chair, but say nothing to them (talking is giving them attention for the bad behavior). Keep repeating this until they sit in the naughty chair (or very very very very close by - use your best judgement), for the full time allotted.
~remind them why they were put into time out and ask for an apology (if the 18 month old cant talk, remind her why she was there, and ask her if she is sorry - hopefully she will be able to say yes or no).
~After the apology, give a hug and allow them to get up from time out and resume their activities.
~hold no grudges, and move on with hope (this does neither of you any good, and by holding on to negativity, the children may act up even more...)
~most importantly, remain calm during all this (even if you want to scream inside). By doing that, you're showing that you're in control, and that they simply must behave, and its not going to hurt YOU if they're in time out (it is just hard on them). If they see it is upsetting YOU then they may try to provoke it to watch your reaction.
I'm sure you're doing a great job with them... I'm just sharing some of what has worked well for us. I hope it helps with your stress level!
as for the running off in parking lots, how about keeping your 3 month old in a sling, putting the 18 month old in a stroller, and having the older one help you push? I know it sounds like a lot of hassle, but it may end up saving you some stress in the long run, and it really is only for a short while, going from a to b.
