Help me be a good mum?

I'm going to sound like the worlds worst mum, but i need advice, so here goes...

I have three children, 6, 5 and 3.

Individually, they are great kids (maybe with the exception of 3, who is extremely demanding, and strong willed. But can be a darling too) i love them incredibly.
I'm sure many of you can relate, when i say, put them together and the sh!t storm ensues.
I am not a patient mum, i'm sad to say. I snap quickly and i hate myself for it. Nasty things come out of my mouth before it registers in my brain.
My children love me, i know they do. But i don't want to be she nasty shouty mum. It doesn't work.
With the holidays upon us, i feel sick at the thought of having all three with no break. Other mums cant wait!
I need ideas to motivate them to behave well, to take care of their belongings, their siblings. The fighting is the worst, 6 and 5 winding 3 up, the screaming... I can't handle it.
They really arent bad kids, but when together...
I sat on my bed and cried for 10mins after the diner time saga.
I just feel like such a crap mum.