Yes, these three concepts are different from each other.
The YouTube username is one of the first things you pick when creating a channel. Once you gain 100 subscribers, you may change it.
The YouTube legacy name is no longer available, yet older YouTubers have it. You can notice that these people have /user/ item with their username in their YouTube URL.
A YouTube channel ID is a unique identifier. It consists of 24 characters assigned to each page.