Hi Furkan! These three words are all very similar, but can be used in different contexts. I agree with the first response - it is hard to explain the differences. "Interior" usually means within a physical space. Example: "I want to paint the interior walls of the building". The word "internal" is similar, but it doesn't have to mean a physical space. Example: "Send an internal email telling employees about the new policy.". In this example, it means the email should only go to people that work for the company. "Inside" is the most flexible of these three words. It can be used as an adjective, but it doesn't have to be. Example: "Let's go inside". This is a long way of saying that in some specific examples, yes you can use all three as an adjective interchangeably: "Let's paint the internal/interior/inside walls". But in other cases, one would make more sense and the others would not.