It's something you really should ask your own pediatrician, because sometimes it depends on the baby's weight and/or how hungry s/he seems to be.
Having said that, I've known lots of people who feed their babies baby rice cereal at 3 months or even before. The other cereals for babies may be too "strong" for a small baby's digestive system.
Something to know about rice cereal is that it's binding. People I've known who fed rice cereal also fed their babies a small amount of a "mild" strained fruit, like applesauce. Formula fed babies who have iron in their formula may have a tendency to get "irregular" and/or have "belly problems", so adding a binding cereal might not be particularly "great" in that way.
You probably know that any foods should be introduced one at a time, with a few days between each new one, in order to be able to determine whether it seems to disagree with the baby.
Introducing foods too early can place the baby at a higher risk for food allergies, so that's something to keep in mind too.
This is why you should ask your doctor about it. Whether your baby is breast or bottle fed, whether there's iron in formula, whether your milk may not be quite enough for the baby, whether your baby seems to require more calories, etc. etc. - all part of the picture, and all things best addressed by your doctor.