wow! this is SO my life right now!! i am so sorry to hear we're going through the same thing (and also kind of glad i'm not alone!!). DD's tantrums in public are insane (she's also almost 3). DS is 11mo and crawling/walking, so wrangling one while trying to keep the other from hurting herself thrashing around is almost impossible.
i've started doing this process when she has a tantrum in public. once i see it starting, i
(1) deal with DS first. put him on my back in the ergo, in the stroller, occupy him with a toy. you think that you should deal with the tantrum first because it draws the most attention, but it's actually easiest to calm yourself by taking care of the one not going insane first! gives you a few moments to collect yourself before you deal with the tantrum. also, it keeps the other one contained so that you can give your full attention where it's really needed at that moment.
(2) i get down and ask her if she'd like me to help her obey.
(3) inevitably she says no, so i tell her she has one more chance to [blank] on her own before i will help her to obey.
(4) when she screams/cries/etc 'no' again, i pick her up and tell her i will help her to obey (which generally involves first exiting the center of the room/middle of the aisle/jungle gym, first).
(5) if our destination is the car, i ask her if she'd like to walk on her own or have mommy carry her. inevitably she repeats that she wants to do whatever we are leaving, so i tell her that i will carry her to make leaving easier. of course, it doesn't because she kicks and screams, but it helps me because by this time i am pretty frustrated and being required to carry her in a big bear-hug embrace (to keep her from squirming out of my arms) is actually a good adrenaline release for me. i'll talk quietly to her the whole way to the car even if she's screaming and by the time we've arrived at the car usually the tantrum has simmered down to just crying and clinginess.
i don't know how well this works, but at least it keeps me from screaming too!!
i'll agree that nobody ever offers to help me, not even hold DS or anything. maybe it's just the negative self-talk in my head but i feel like everyone is watching me and wondering "why can't this woman control this crazy child?"
i just want to know when this is going to end! will age 3 be easier? i feel like if i could just know it was going to be over in a few months i'd be able to hang on a little bit better. it's so much easier if you know the end is in sight!!
hugs to you from a mama whose daughter is probably having a tantrum at the same time yours is!!