Waxless toilet seal - measurements

Anyone have any experience with Fernco's waxless toilet seals? Do they work as well as promised?
Here's my dilema. The outlet on the bottom on my toilet meaures 3 inches, and the inside of the flange in the floor measures what appears to be 3 1/2 to 3 3/4. (I probably have some leftover "gunk" from the old wax seal to chip away at...???) Either way, it's bigger than a 3 inch hole, and smaller than a 4. Fernco makes a 3 inch seal...a 4 inch seal...and what they call a "combination" - that fits a 4 inch toilet to a 3 inch flange.
If I use the 3 inch, will I get a good seal in the flange?
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The controllig thing is the pipe size. Should be either 4" or 3". The size of the flange is not important. The semi rigid flanges on the waxless seal fit down inside the pipe makeing the seal - it does not seal to the toilet flange.
Harry K
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