Is it obligatory to feed or clothe, or to fast three days as an expiation for an oath, knowing that there are several due expiations and that the questioner only possesses $300 and has no source of income?

It is not permissible to resort to fasting as expiation for an oath unless one is unable to provide food, clothing, or emancipate a slave. So, if you have surplus funds beyond your basic expenses sufficient to feed or clothe ten needy persons, then fasting is not permissible for you as expiation. You would only expiate by fasting for those other forms of expiation that you are unable to fulfill, if the amount you have is insufficient for all of them.