I have two girls (3,1.5) and a five week old baby boy. Not to compare too much but he seems more hard work at times that those were. He's a bit fussy with breastfeeding, can have a spell of a painful crying noise (maybe colic?) and doesn't seem settled sometimes. Also feeds more frequently but was born bigger and is a big boy...
I can't help but think it's just the fact that he is a third baby in a close age gap. I have one less pair of hands, two other childrens needs, and I'm always busy (school runs, groups etc) so I never have chill out time with him like I did the girls. He's always being interrupted with his sleep for getting in and out the car on school runs, being out an about. Plus poked and prodded by bigger sisters! 
So is it all because of he's a third or is it just that he's very different? I really do think it could be all these factors.. He gets over tired from it all and then doesn't want to settle.
I just want to have the time to breathe, embrace it all, enjoy quiet time with him like I did with the girls. It was a lot less stressful with them because I had no deadlines / time limitsx