[archduck]

In 1) and 2), it's clear the speaker is making some kind of conjecture. In 3), I think there's some bleed-over interfering from the idiomatic "I guess" which indicates that you're reporting hearsay or hedging your commitment to its accuracy. Some languages express this "evidentiary" modality more formally, e.g. via some kind of morphosyntactic change that makes it explicit. English has a ton of ways of expressing modalities, but they're wrapped up together in various kinds of constructions that express tense and aspect as well ("TAM" is an acronym for tense-aspect-mood used in lots of linguistic analysis across all kinds of languages).

Usually it's an error to add -ing to a mental process (Are you knowing the answer?, I'm believing in a deity, etc.). (We can choose to construe a mental process as a material one - it's not a lexical rule about those verbs, but a grammatical means we have access to, e.g. I'm lovin' it, I'm thinking about you contrue them as an activity rather than a mental state.) But perhaps the reason why replacing "guessing" with "guess" changes the meaning so much is because "I guess" is already taken. It should modify the meaning as "I think x" => "I'm thinking x" does, but saying "I guess" sounds like a way of hedging information from a 3rd party.