Missouri can tax only the income you earned on the three days you physically worked in Missouri. The income you earned by working remotely in Minnesota is MN-source income, not MO-source income. Work income is "sourced" where the work is actually (physically) performed. MO taxes non-residents only on MO-source income. The employer's location does not determine sourcing.
This MO tax website explains the rules on whether or not you're required to file a MO tax return:
https://dor.mo.gov/taxation/individual/additional-resources/non-residents.html
Note that MO requires you to use the same filing status on your MO return (if you're required to file) that you used on your federal return.
ALL you & your husband's income is taxable by your home state of MN, regardless of where you earned it. It is almost always advantageous for married couples to file jointly.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
