I think - based on what is in your mind at that moment. You're using what you know, but it's not a definite conclusion. You'll have to explain yourself.
I guess - you're using a bit more imagination. This can mean you're not so sure, but at least you don't have to explain yourself. Apparently the etymology refers to taking aim at a target.
I reckon - your conclusion based on what you know. In the neighbouring language Dutch, "een rekening" is an account, an invoice or a calculation. You get the idea. :) In English , it's used more colloquially. Not only central south USA uses this, Australians often use it too.