Any capital gain you have would be long-term capital gain (obviously).
You have a basis, but it is complicated. It appears as if you received a gift of a one-third interest from Owner #1 and, in that event, you would have a carryover basis (e.g., one-third of Owner #3's adjusted basis).
When Owner #2 passed in 2016, you and Owner #3 acquired Owner #2's one-third interest (one-sixth each), the basis of which would be the fair market value on Owner #2's date of death. You and Owner #3 each had a one-half share following Owner #2's death.
When Owner #3 passed in 2021, you acquired the remaining one-half interest, the basis of which would be the fair market value on the date of Owner #3's death. You would then, typically, own the entire fee in severalty (i.e., you were then the sole owner).
You can clearly see that this is complex and would require determining the fair market value on two dates in order to determine your exact basis. Since the sales price is relatively low, you could consider doing some research yourself into the values on those dates.
