3 month old just isn't eating much and is fussy.

Sickness: Fussiness, less eating /poor sucking, excessive sleepiness, lethargy with poor muscle tone, fever or simetimes hypothermia are signs of infection. There can be associated runny nose, nasal stuffiness. Feed every 3 hours, make his nose is not blocked (b/c up to 6 mo. They are obligatory nose breather), make sure he pees at least 5-6 times/day. If any worsening, see his pediatrician.