**Answer:** 2 eggs.

We are told that:

1.5 hens will lay 1.5 eggs in 1.5 days.

We can double the chickens (which doubles the eggs):

3 hens will lay 3 eggs in 1.5 days.

We can now double the days (which doubles the eggs):

3 hens will lay 6 eggs in 3 days.

We can now divide the number of hens by 3 (which also divides the numbers of eggs by 3).

1 hen will lay 2 eggs in 3 days.

QED.