PayPal has differnt interface for handling recurring payments and normal cart/buy now payments so recurring payments cannot be added to the shopping cart.
I would create 3 different subscription buttons as follows and put them on the Membership payment page to accept the payment:
Button 1 [$6 per month]
Button 1 [$7 per quarter]
Button 1 [$11 per year]
Let me know if this helps.