Sorry, but when I read your question I did laugh to myself. Um, Im sure there are people out there who didn't have any trouble with the 3's. I don't know any of them, and if I met them I'd try and find out there secret!
Fact is, when toddlers turn 3, there is such a huge change in independence and wanting to express themselves that it can be exausting. A friend of mine has a 7yo and a 4yo and when we were both talking about having a third, she said "I want another baby, but I don't want another 3 yo".
When my little guy was three I found the days to be long and frustrating. He challenged EVERYTHING. For example, this was a typical exchange:
DS "mom, can we go to the park today?"
ME: no love, it's raining outside.
DS: no it's not raining.
ME: yes it is, see all the raindrops coming down and hitting the ground, that means it's raining.
DS: no, it's not raining.
DS: I WANT TO GO TO THE PARRRRRRRRRRRRRRRRRRKKKKKKKKKKKK it's NOOOOOTTTTTTTTTTTTTTTTTTTTTTT raining!!!!!!!!!!!!!!
(Previous to this, redirection worked really well...however, my three year old decided that nothing was going to distract him from being frustrating).
ME: Ooooh, look at the beautiful crayons we can color with.
DS: Crying, screaming looking outside. Will not answer me.
This was a fairly typical daily scenario. You're right about the second child, my dd was born 10 days after ds turned three. Although he showed no jealosy, I found that when he screamed and cried like this, I would get more frustrated with him because she'd be sleeping.
I can remember posting a thread titled "Does anyone want to lock themselves in their bathroom rather than deal with their 3 1/2 year old?"
I was serious. And, I got some amazing book suggestions, one of which is "Your Three Year Old" by Louise Bates Ames. I thought this was such a fabulous book, I bought the whole series on Amazon.
Anyway, three is typically a difficult age, but everyone is different and your dc may choose another age to show his independence and challenge your authority. The important thing to remember is that it's normal and most children go through it.