*Note: I live in the Southern United States so my English may be a bit different from what you're trying to learn but I will try to use as standard of English as I can. To suppose is to suggest that you possibly could do something, but you're hesitant to do so. "I suppose I could let you borrow 5 dollars, but you have to promise to pay me back". Believe is used to indicate that you think something is true, especially if the belief is personal, faith or religion based. "I believe that Tom will make it back home alive". Think can be used to describe or catalog any of your personal thoughts. "If I don't stop thinking about Veronica I will never get over her".

I think so = 100 % I believe so = 50% I suppose = you are guessing for example you can say I don’t know but I suppose so