The best advice I've heard on here was imagine you are on cctv/being filmed for a parenting programme/reality tv.
I just think what would my friends think if they could hear me?
I find if you feel things spiralling it's helpful to switch the mood. So I say that the behaviour is out of order, but then I'm all singing all dancing dynamic mummy who suggests junk modelling or a HSM dance off (this is not in my nature but I do have to do it in emergencies).
This may sound terrible, but when my twins were tiny babies and ds1 was 3(he is autistic and was a handful then) I dealt with the twins needs-breastfeeding and so on, but I didn't pay them much attention when I had ds1 after preschool.
The babies had me in the night, when ds1 was at preschool and the evening, and I just felt that ds1 (who was wigging out big time) needed my attention more.
So I could be feeding them/changing nappies, but I was always talking to ds1. I prioritised him-baby in a stinky nappy and ds1 moaning about something-I'd go to ds1 first.
When you're dd comes home from school it's her time. My lot can be vile with tiredness after school, but if I've sorted snacks, tea and put an activity out (just paints or something) it goes a lot better.
I know bad behaviour outside the home is excruciating.
I try to adopt my dh's attitude and not give a stuff about what people think. I used to just take ds1 home if it was all going crap-no point in persevering with an outing if no one's enjoying it. Or if you have to stay then give baby to willing hands/put in pram with dummy and focus entirely on dd and engaging her in something positive.
If it's adhd or if she's 'brushed' by it (ds1 has a definate dx of aspergers, but other family members are what I'd call brushed by it) then that is very hard work. Have you looked into PDA? With my friend's adhd little girl she finds distraction the best form of behaviour management.
Sorry for rambling on.
Glad you're feeling a bit better.
You need a good chat with dh,gp,health visitor and friends.
I would feed twins then hand them straight to dh (at weekends) and take ds1 off to the park.