I agree with tagteam that it appears that you have a gift in the purchase, 1/3 of the purchase price, $10,833.33.
I am assuming JTROS since nobody else comes into play when someone passes. This means, when owner #2 passed in 2016, you and owner #1 split the value on inherited date. For example, if the property was worth $39,000 then you and owner #1 would each increase your basis by a third, $13,000.
In 2021, owner #1 passes, you inherit the rest of the property at the value on that date. For example, if the property was worth $44,000, then you would inherit 50% of the value, $22,000.
Check into the values and get the correct numbers for date of death for the other owners.
If others inherited, that changes the rules.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"
