Re: Sorry, missunderstood when reading:
I'm not sure about what you actually asked, but I'm not honestly aware of any Standard Size Dryer's ever having a 3-inch Vent opening, "verses" a 4-inch opening. I just know that as mentioned the Compact/Portable Dryer's had a 3-inch Vent opening, but even with those, I'm not sure when they changed to the 4-inch openings, but I would guess somewhere around the Early-1950's. My 1950 Kenmore Dryer that I just acquired i believe has a 4-inch Vent Opening on the back upper right side, I will have to check it out to make sure. I'm sure no doubt that one of the other Club Guy's might know for sure. Are you able to convert your Vent thru the Wall?
Peace, Good Luck and Happy Drying, Steve
SactoTeddyBear...