Within the last three months all the three toilets have become clogged and needed plunged. We have not had a problem with any of these toilets in the four years we have lived here. Nothing has changed. They even plug and need plunged if only water is flushed. Could a clogged vent line be the common problem?

Are there any bubbles or gurgles in the toilets? Do they back up and run over? Back to you. Tom

No they usually do not run over. No gurgles or bubbles. No pattern either. Couple of swift punges and they drain. Sometimes we know it is going to happen since the bowl water level is very low. Two of the toilets are on second floor and one is on the first floor. We have always used Scott paper since this is a very old home. (1918).

City sewer or septic tank? I would also check the vents. If they are blocked that could stop the siphon action, Regards, Tom

City Sewer. No other problems with showers or sinks draining. Only one vent stack and it is on the roof of a three story home.

Sounds like you were correct when you asked," Could a clogged vent line be the common problem?"
You're revented in the attic out to one roof vent. This means that any snaking will have to be done in the attic and the local vents will have to be disconnected from the roof vent and snaked on a individual basis. Not a job for a home owner. Good luck, Tom