It depends on a lot of things. Such as filtration, hiding places, water changes, the species of pleco etc. What is your reason for wanting to add the pleco?
i just got one for free but im not sure where add it since my cichlid tank is the only option for the winter but i dont want my cichlid killing my pleco
I didn't catch the "common" part in the title. A three inch common pleco will quickly outgrow that size of tank, even if its just for the winter and the waste will likely be more than a 30 gal tank can handle. IMO the best place to add the pleco is back to where it came from. Common plecos really shouldn't be kept in aquariums. They reach a size of 2 feet and aren't suitable for what most people said. That and they don't really even eat algae after about 4 inches, just left over fish food and fish food that the other fish won't get.