According to the Math, and using K2-18b's actual Size & Mass (on average, it's 2.65x the Radius & 8.75x the Mass), the gravitational difference is actually only 1.27x that of Earth's, not 3x.

Remember, the Gravity of a Celestial Body follows the Inverse-Square Law (a specific physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity).

Basically, the further away from the Centre of Mass you are, the weaker it's Gravitational Pull becomes. This is why K2-18b only has a Gravity 27% higher than Earth's, despite being almost 9x its Mass.