You Gain weight because If the mass of the Earth became 3 times what it is today but the Earth remained the same size, then your weight would also become 3 times what you weigh today.
Weight is the measure of gravity pulling us to earth, whereas mass is the amount of matter that makes us. your mass is the same on earth and on the moon, but your weight goes down on the moon since there's less gravity. So, your weight will increase if the pull gets stronger. So say the pull gets triple stronger, then your weight will be tripled.
