Bloody hell OP, you have 3 pre-schoolers, you deserve a medal!
You need to give yourself a break (emotionally I mean).
I only have 2 DCs, two years apart in age, and they are both at school now thank god - and like you, I used to have a full-on career and found the switch to motherhood incredibly hard; the kids nearly broke me.
Here are my tips:
Raising children is the hardest job in the world. You are raising three, all day, every day .... If this were an office job, and you were struggling, you would look for ways to make things easier and more efficient. Get a cleaner. Look into some childcare for your eldest - presumably she's entitled to 15 hours per week at preschool? Or can you find a mother's help / daytime babysitter for a few hours per week?
When the younger 2 nap, put your feet up.
Get outside in the fresh air every day - presumably you're walking the dogs anyway, so extend the walking time and let the kids run around and tire themselves out. It doesn't matter if they get wet/muddy!
Housework can be shared with your husband/partner in the evenings and at weekends because LOOKING AFTER 3 YOUNG CHILDREN IS A FULLTIME JOB IN ITSELF!!!!
Lower your expectations. Then lower them again.
Finally: your friends are NOT doing everything perfectly. They are just faking it when you see them. There is a conspiracy of silence amongst mothers - we never admit to people in RL that we are struggling because we are afraid of looking like failures. Thanks god for Mumsnet.