If the heaviest isotope is more abundant and the other two isotopes were kess abundant what would happene to the atomic weight of beanium

If you have one bucket with a few iron balls and a lot of cotton balls, and another one with a lot of iron balls and a few cotton balls, which bucket would be heavier?

Ofcourse the 2nd one.

The atomic weight would be close to, but less than, the weight of the heaviest isotope. It will shift towards the heavier magnitude.