"I suppose" is roughly equivalent to "I guess." In most cases, all three can be used interchangeably. As responses, they all end up being: "I suppose so" ; "I think so" ; "I guess so." They all mean that you agree that the speaker's statement is probably accurate. In the same way, they can all be used to describe something you are not entirely sure about, but want to express to others as a guess or logical argument. "I suppose" implies some sort of logical thinking and conclusion, perhaps drawing from previous knowledge. It can also be a reluctant agreement to the other speaker's statement. "I guess" implies that you are just making a guess based on some knowledge, though you may not be entirely sure. It can also be a reluctant agreement to the other speaker's statement. "I think" can be similar to "I guess", or it can mean something more like "I believe". It can be a personal belief or something that you thought was true before this conversation and are now stating it. The other two imply that you are thinking about it while you speak or listen, "I think" would generally mean that you had thought about it before, unless it is clear from context that you are guessing.

I can "second" Cameron's answer... I would say "I think" and "I suppose" are pretty much the same.