(Background info: I am the oldest of three by 13 years. My baby sister Lucy was born when I was 13, so has just turned 5. My little man (brother) was born 16 months later- he's 4 today actually :D )
It wasn't easy for my parents at all, it was made even worse by the fact my mum had to have a C-section with my brother- so she had to be pretty careful for 6 weeks after he was born- not easy with a toddler!
Luckily, there was the two of them, me, and a multitude of other relatives willing to help out, so they could get time out if they needed it.
Now, things are quite a bit easier (except that they are so competitive and Lu has such an advantage over him)- and I know full well that no-one regrets the situation.
The only one of all three of us that was planned was my sister. I was an accident at 19 (something we can laugh about now), then there was years of fertility problems, my 'miracle' little sister, and then the 'surprise' that was my brother.
It probably depends on your situation- my parents had so much support (and a live-in babysitter- me) so things were fairly easy for them.
Love my little brats to bits though!