My 3 month old baby randomly sucks when nothing is in his mouth?? And is drooling a lot, what can this mean?

Self soothing. Babies are driven to suck as a survival tool. If they didn't, they wouldn't learn to feed as a newborn when they have never experienced hunger-ever. Sucking is rewarded by pleasure sensations in the brain, and an infant with few capabilities can turn to it for pleasure at any time while awake (& partly asleep). It is why nail biting, gum chewing and smoking are difficult habits to break.
Babies love to suck, so it's not unusual for your baby to suck even when nothing is in his mouth.That sucking can lead to increased drooling also. Many babies do start to drool at 3 months of age, even if no teeth are coming in,but you should also check for teeth as a possible cause.If your baby is healthy,alert,feeding well,I wouldn't worry about either of these things;if not,or if you are still concerned,see his doc.