I don't see how that could work Keke. Your 30-minute classes cost $5, but your 90-minute class costs $9, (which are already very low prices!)
If the student is asking to pay $3 per 30 mins., I would definitely say no. You can't be expected to plan for and teach 3 x 30 minute classes at a rate of $3 each!! I would advise you to stick to your advertised prices...🙂