I heard that there were 7 nations in Israel,.
why Shemos 3.8 only mentions 6?
The Girgashi are missing from the list in Shemos 3:8, like you mentioned.
Rabbi Naftali Zvi Yehudah Berlin in Haamek Davar poses the same question. Here's what he said:
והחוי והיבוסי. ולא אמר הגרגשי כאשר אמר אל מקום ולהגרגשי לא היה מקום מיוחד אלא מפוזרים בתוך שש אומות. ומש״ה לא היו תופשים מקום וכשראו שישראל נכנסים לארץ עמדו ופינו לגמרי מחמת שגם בארץ היו טפלים להאומות אשר המה בקרבם
Translation (special thanks to hazoriz):
The Chivi and the Yevusi. The Girgashi are not mentioned because the passuk is talking regarding a place, and they did not have a unique place (a specific land) but were spread out within six nations. And because of this they did not grab a place (land), and when they saw Bnei Yisroel were entering into "The Land" (Eretz Yisroel) they got up and emptied completely since they were already secondary to the nations they were within.
Ibn Ezra says they were the smallest of all the nations, which is why they were not mentioned: http://sefaria.org/Exodus.3.8?lang=bi&with=Ibn%20Ezra&lang2=bi
Ramban on the verse says that they are not mentioned because their land was not blessed with milk and honey