<sup>Yet again I beg your pardon for my inability to create proper tags.</sup>

These are the three most common ways to say "I think." (At least, I believe so. I mean, I think so. Um...)

Are there any subtle differences between them?  
Are there situations where one of the three is more suitable than the others?  
Can you say you hear a particular form all the time in the streets, while the other two are much less common?