ようと思う ➡️ つもり ➡️ 予定

Showing a desire to do something, from least sure to sure that you will do.

A scenario I’m imagining:

Someone asks me if I’m going to a social gathering.

1. 行こうと思う I think I’ll go. (but I might not go as well)
2. 行くつもりだ I intend to go. (stronger than 1, but if something else pops up that is more important, I might not go)
3. 行く予定だ I plan to go. (I’d probably already put it on my Calendar, to make sure I don’t forget to attend)