As mentioned above, the gemara is establishing a basis of measure independent of the physical fact that the circumference is more than 3. Once established, this will be the unit of measure to be used when questions of this nature need resolution. The Torah is the source of nature itself and supersedes facts as we know them.
It is interesting to note that in the following few blatt much discussion revolves as to whether 2.9 or 3 tefachim is lavud. This is the basis of a whole machlokes.
Assuming an amah is say 22 inches, 1 tefach will be 1/6 of that or 3.67 inches. 1/10 of that is .37 of an inch and this forms the basis of many blatt of machlokes and no one questions as to whether this difference in actual fact will prevent a goat from slipping under a gap.
The Torah tells us when to make these minute distinctions and when not to.
Do not panic, I will not embark on a whole discussion on the Brisker shiurim by the seder for tonight.
One shiur is important to remember.
There is no limit on how much to love, honour, and respect your spouse and children.
Chag kosher and someach to all of Klall Yisroel and may we be zocheh to see the end of this long and terrible golus so that Eliyahu Hanavi will finally come and take the Yidden out of golus and the golus out of the Yidden.