According to Ibn Kaspi, "לא איש דברים" means that Moshe was not an eloquent speaker, he was literally not a man of words. This was relevant because God was asking him to be a leader, and good public speaking skills are often thought of as crucial to such a role. No one would ever get elected president or prime minster if they couldn't deliver a good speech.
"כבד פה" and "כבד לשון" on the other hand, are actual speech impediments. (According to the R' Chananel quoted in the footnote in the link, "כבד פה" would refer to the letters זרש"ץ and "כבד לשון" to the letters דטלנ"ת.)
Now, "ערל שפתים" could refer to a third speech impediment (which would be the "lip letters" בומ"ף) like the opinion of R' Yitzchak Karo quoted in @msh210's answer and what seems to be the opinion of Abarbanel, but since Moshe didn't mention it originally when talking to Hashem like he mentioned the others, I prefer the Ralbag's understanding, that it means simply that Moshe is not a good speaker in that he often has trouble expressing himself through speech, often has trouble finding the right words to express the idea he is trying to convey. In this respect, "ערל שפתים" is kind of an antonym of "איש דברים".