Because there are more chances of a successful pregancy that way. Usually at least one will die.
To improve the chance of conception
With 3 eggs there is more chance of one of the eggs attaching to the womb lining...IVF doesn't always work because sometimes none of the eggs attach...hope this helps
It is done to increase the probability of conception. You can read more about IVF and its need. This blog explains wonderfully what is IVF treatment and why is it required.