In WidgetNote these are the same feature wearing different clothes, and it is worth saying that plainly rather than pretending there are three products. What changes is the shape of the note and how you use it. A post-it is the smallest form: one square, one line, a bright paper color, usually tilted a few degrees. That is a styling choice you make in the app, not a separate mode.
The sticky note widget page covers the same idea with more attention to the styling controls — paper, font, color, and readability against different wallpapers. The notepad widget page covers the opposite end: a larger widget holding a running list that keeps growing through the day. Same note engine, different job.
If you are choosing between them, the deciding question is whether the note is finished when you write it. A post-it is finished — a phone number, a door code, one instruction. A notepad is not — you will add to it four more times before the day ends. Picking the wrong one is the usual reason a Home Screen note stops getting read.