Your circumference is actually 76.65mm. Therefore one full revolution of rotation gives 76.65mm of linear motion.
You are most likely using 1.8* steppers. That means there are 200 steps for a 1:1 in rotation. If you are running 1/16 microstepping…

Disable stepper motors. Then position the carriages so they are all exactly the same distances as measured from the bottom. The run the G132 S1 command. Offsets are measured and saved in eeprom.

For a Cartesian printer I would say your steps per mm is off. With a delta I believe it is the diagonal rod length that is off.
What is your current steps per mm? What size pulleys are you using?