I would never CIO such a young baby. Honestly, 30 minutes of crying for a 3 month old is a lot. I know some babies are collicky but your baby simply didn't want to be alone and you ignored her request (at least that is what she was thinking).
My 8 1/2 month old falls asleep nursing (in bed with me) or in the baby swing or in the car while driving. We have an ocean sounds white noise feature on an alarm clock and we have ocean sounds in our bedroom for many hours every day. When My baby gets overly tired and I know she needs rest, I put her in the baby swing wrapped up tight (similar to Happiest Baby style)and with a pacifier in her mouth, with some loud ocean sounds going and I (or DH) swing her manually, while smiling at her, singing songs, kissing her feet when she swings near us. More often than not, she falls asleep within 2 minutes.
I sort of think it's a mistake for you to take away her walks if she doesn't fall asleep. She has just gotten older and is more interested in looking around. The fresh air is good for her, and will only lead to tuckering her out more.