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.