My baby is 3 months old and has reddish rashes on her cheeks, elbows, and knees. is it just eczema?

Probably, but: I would not blow it off. Many infants are allergic to a food that is causing the rash. Getting an allergy evaluation to determine if a food is related to the rash may help to dramatically improve the rash once the food culprit is eliminated from the diet.

It could be: Eczema on the arms and legs is more likely to be on the inside of the elbows and the back of the knees. The face is a common site for eczema in infants.