Staying calm and patient is incredibly, incredibly hard. Mine are 10 and 12 now and I find discipline is actually easier (although I suspect that may also be due to them having learned boundaries very young) now they are older and have well developed conscience and empathy. With a stropping 3yo it's incredibly difficult. My mum, who did occasionally smack me and my DSis comments on how patient I am - and how she wishes she had been able to do the same. I don't know how I've done it, it's just in me.
And then I resort to reading crappy vampire romances when the DDs have gone to bed to wind down. Somehow it works.
I've had to resort to some pretty unorthodox methods - such as the day DD1 (5 at the time) refused to put on her shoes. So I said fine, go without (this was in February). Knowing full well her PE kit, including plimsolls, was at school, I took her to school, walked with her from the car to the school gate, explained to her teacher what had happened and could she please wear her plimsolls for the day, and left for work. I did not at any time raise my voice - no idea how, I was seething inside. But it worked. She never tried it again.
Funnily enough I now have two DDs who will put themselves into 'time out' when they feel they are getting overheated - they simply tell us they want a time out, go to their room and cool down. It's taken a long time, but it works really well.