3.14159265 or just 3? (Eruvin 14a)

Someone pointed me to the Tosafos Harosh who asks the obvious question on today's daf. Why does the gemara need a pasuk to prove how to measure the circumference of a circle? Can't we just measure it? He answers that the pasuk tells us that we are allowed to estimate it. The gemara is questioning how does a circumference of 3 mean that the diameter is 1? We know that the circumference would have to be more than 3 to have a diameter of 1!?! The gemara answers that we see from Shlomo that for halachic purposes we can estimate pi to be 3 even though we all know that in reality it's more than 3.