- Ultimate seal is a flnge that goes inside pipe. That should never leak.
- Though I haven't seen it stated anywhere - don't fit a flange outside of smaller pipe (3") - always inside a larger pipe (4") - so even if the glue job leaks, there will be no external leak between the flange and the pipe. Any leak when flange is inside the larger pipe will still go down the drain.
Since shitters definitely get clogged - it is probably a better idea to use a flange that goes inside a 4" pipe, not inside a 3" pipe (those exist). Safer yet - in terms of more space - use a 4" spigot flange - meaning that:
- Knockout on toilet flange is for testing, to seal the toilet hole.
- Or, you can get a toilet test plug - 
- 4" plug - goes inside a pipe, or seals threads of cleanuot? 
- Note - plugs typically seal inside of pipe. A spigot flange means it goes inside a 4" fitting, ie, the 4" spigot flange will have a size the same as OD of 4" pipe. But the ID of a spigot flange - will that be 4"? Hopefully - but cannot guarantee that a 4" plug will work with a 4" spigot.
- See description of various flange styles at bottom of page - hub or spigot or inside/outside pipe connection, either open (rework) or closed (for new construction + testing)