what part fills up? if the bowl fills up like its gonna overflow you prob have a main line that is clogged. is this a single story house? did they all start doing it at the same time? any small children that are missing toys?
Do you have a clean out in the front yard? If yes twist lid off and somebody flush toilet is water moving pass the clean out? If no snake from clean out to house. If yes snake from clean out to main. This is assuming your not on a septic tank. Your best to snake both ways. But this will determine where the main problem in the pipe is.