Kate,
When Cate WAS constipated at 6 months (that bloody rice cereal) and she was clearly in need of help (she was so uncomfortable poor baby), a health nurse suggested "tickling" her bottom with a cotton bud dipped in Vaseline. (I had heard similar reports from others using rectal thermometers!). You just twist it a little........not too far......
Ahh the things we do :lol:
Anyhow, about 20 minutes later............it happened.
If you can stomach it, may be worth a try.
Also, tummy massages and leg lifting (they can nearly bend in half at that age!) seemed to help!
If he was a little older, I swear by prune juice; but not for a 3 month old.