Am I considered a "resident" if I'm an international college student of three years and had never filed Form 8843?

@yunchi-liu , generally an international student with F-1 visa  is considered an " exempt " person for  purposes of "Substantial Presence Test " for FIVE calendar years starting from the date of entry.  Thus , absent any other precluding facts like earlier admissions / stays in the USA , if you entered the country in September 2020, you would be an exempt person  through the end of  2024 . During this period  i.e. 2020 through 2024  and unless your status is changed, you are a 'Non-Resident Alien" and report your income on form 1040-NR, irrespective of  form 8843 actions

If you need more on this please consider  providing the following info --  was  this your first entry to the USA and if not  need dates of entry and stay length during the last five years, citizenship country, if married  then to whom ( US citizen / GreenCard  or Non-resident alien ).

pk