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.

Use "I think" when you have a definite idea about something. Use "I guess" when you are not so definite, have some uncertainty, or don't care much one way or the other. Use "I reckon" when you want to sound like a cowboy, or someone from Texas or Oklahoma.

Haha I like "I reckon": "You're the boss here, I reckon." (with blood all over the face) "Yeah, you got that right, you pathetic m-f."