You can give a discount on however many subsequent payments you want as long as it's a manual renewal. Recurring payments are the only thing restricted to first payment and second payment (recurring payment). To do this for manual payments you're just do what you've already done with your promotions except add more of them. Starting from my example below.
www.joomlapolis.com/forum/developer-members-support/248028-help-implementing-options-for-diffent-discounts-for-month-1-and-2#343799
We can add a 3rd to this so the 3rd renewal is discounted differently. Example as follows.
Promotion 3
Promotion Type: Applies to all purchases
The promotion is: R*P: A percentage R of the item price P
Discount Rate in PERCENT (%): 15
Maximum uses per customer (0=unlimited): 1
To which plans this promotion applies: SELECT_PLAN_HERE
The key here is 3 things. Priority, Order, and Maximum Per Customer Uses. Ensure they all have the same Priority (e.g. 1, 3, 10, etc.. whatever you want). Next ensure they're ordered from top to bottom. So Promotion 1, the biggest discount, must be ordered before Promotion 2 and Promotion 2 before Promotion 3. Finally be sure "Maximum uses per customer (0=unlimited)" is set to 1. This should then allow it to continue down the chain of promotions until they're exhausted with each manual renewal.
This is not possible with recurring payments. You can only discount the first payment and/or the recurring payment. That's it. Discounting the recurring payment will discount it until they decide to cancel.