Baby's uncomfortable. A 3 month old baby may cry (around other babies) because his feelings of comfort and security got interrupted. Maybe he is no longer being held by mom/dad, as he got placed down next to another baby, or maybe mom/dad changed her/his tone of voice. Maybe the other babies are crying and being too noisy. Perhaps the other babies are making random hand/face gestures that are look too stimulating.
Overstimulation. It could be, if there is a lot of noise, that your baby gets frightened by the noise. It could also be a separation anxiety particularly if your baby associates other children with you leaving him/her.