If commission is a component of your basic salary, and not an incentive, it should be part of your maternity leave pay.
Your maternity leave salary will be based on your average monthly commission for the 6 months before you go on leave.
Your base salary is $1,000 per month, before commission. In the last 6 months before maternity leave, you received three commission payments: $1,500, $1000 and $500, for a total of $3,000.
This means your average monthly commission is $500, and your maternity leave salary will be $1,500 per month.