From an Australian perspective, 'I reckon..' is a very common way of saying 'I think..'. When I ask a friend's opinion about something, I may well ask 'what do you reckon?'

Hi Yuta! All three words are very similar in meaning. 1. Reckon: reckon used in this way is not common in General American (I'm not sure about other dialects of English). I live in Wisconsin, USA, and if I heard someone use "reckon" like this, I would think they were either being silly or they were uneducated. To answer your question, "I reckon that" means that you're not sure, and that you're guessing. 2. Think: think could be thought of as a little stronger than reckon, but to me, they're about the same. Think is what I would normally use. 3. Believe: believe is probably a little stronger than think, but it can also have an emotional element to it.

They have very close in meaning actually. I would explain them as a matter of "gradation" of the belief from 1 to 3: 1. I reckon: it is the result of a process of thinking, a conclusion I would draw after studying some facts presented to me. 2. I think: it is the first answer that come out of my mind when I reply to a question. Mostly used word from the 3 words. 3. I believe: this is strongest feeling. Something that I truly think is correct and you might not be able to make me change my mind. I see a man sleeping in the street, I reckon he his homeless. I think this dress does not match your dress. I believe in you, you can master english.

If I say “I reckon that you shouldn’t buy that.” then that means I came to a conclusion on how or why you shouldn’t buy it. “I think” could be of what you’re not too sure about “I believe” would mean you went through the process of thinking and have a stronger idea of what “you think”