Monthly Instalments in my account

I had a question regarding my apple account. Currently, I have 3 monthly instalments plans with my account which are 3 iPhones. My question was that what happens if I pay early 1 of the monthly instalments if I pay some money extra will the next month be the same money I have to pay I would have no interest in my plans right? For example, if this month I pay extra money for 1 iPhone will the next month be exact the same with no interest?