A.C.
K.,
Congratulations on baby number 3!
My first child was a lot like your 3rd. She had colic for the first 6 months and cried all the time unless she was being held. She typically only slept 2 hours a night (without being held).
Here's some of the things I did: swaddled her as tightly as I could without it creating circulation problems for her, purchased a baby sling/carrier (kind of looks like a kangaroo pouch) to put her in so I was "holding" her and still had two hands available to do laundry, do some light cooking (microwave dinners, sandwiches, etc), while being swaddled I'd let warm water run over the top of her head (trick the nurses at the hospital did), tried a pacifier (didn't work for her, but it might for you), had white noise (vacuum running, static from the radio, anything that after a while would annoy me worked for her), a baby swing/vibrator (swing that rocks and vibrates - was one of our better investments). I also found a support group for moms - something so that I got out of the house a few days a week for playtime, coffee. This gave me some fresh air, other moms to talk to, and usually a couple other hands to help hold, bounce, rock, sway, dance, and talk to the baby - that was a huge help!
Some of the things only gave a few minutes reprieve, others would get better results. As time went by she grew out of her colic state and things got much better! You might want to talk to your pediatrician about it and he/she might be able to suggest some other (newer) ways to calm a baby.
During the first 6 months of my first child's life I too felt like I was going insane. There were days when I felt like I was in a daze and at the end of the day I couldn't tell you one thing that had happened that day other than the baby cried. This was a hard experience for me because she was my first and I had no clue what to do with a baby.
I hope some of the ideas above work for you!
~A. C