My 3 month old daughter cries constantly.  Sometimes, it gets to me where I feel frustrated and overwhelmed.  Every time I try to put her down to rest, she starts crying.  It is rough, and I am struggling with it.  My sister-in-law said that my daughter should start "tummy time" now by laying on her tummy.  That seems to be helping.  She cries when I lay her on her back.  Does any mom have advice how to deal with a baby crying constantly?

I would lay her on her back and make the motions of bicycling with her legs or just push her legs up so her knees are near her chest.  Sometimes if it's gas this helps.  It worked with both my kids.  My sister in law would carry her baby facing out and try to hold him at the belly so there is some pressure there.  Again, this helps with gas.

Also, I know it's hard but if she is crying anyways take a 5 minute break.  Put her in her crib and go somewhere you can't hear her.  She'll be safe there and you can take a time out.  Mommies deserve time outs also and it does work.

I agree with Antoinette. Also try gripe water! It works wonders, your baby could have colic or refulx my son did and did nothing but cry for the 1st 6 months of his life. So it is ok to let your baby cry for a bit, it helps build their lungs as well. I know they say babies need to sleep on their backs and its the only safe way to sleep, but my son slept on hs tummy and I made sure there was nothing else in the crib but him and his blanket. To each their own here. You do what works for the two of yall. Gripe Water works better then the gas drops which made my son consipated. Also when he was 8 weeks old I gave him every now and again prune juice with water, no more then 3 oz. This helped out alot.