If its not a flat tax, what is it?

For an assignment due in a few hours, I have the following question:
Imagine a sales tax rate is 3%. This tax rate is the same on all purchases. The tax is not considered a proportional tax. If it is not considered a proportional tax, what kind of tax is it? Explain your answer.--- It might be a flat tax, though I have found proportional taxes referred to as flat taxes.

Correct. A regressive tax. According to Wikipedia:

A regressive tax is a tax imposed so that the effective tax rate decreases as the amount to which the rate is applied increases. The term "regressive tax" describes a distribution effect, which can be applied to any type of tax system (income or consumption) that meets the definition. It is frequently applied in reference to fixed taxes, where every person has to pay the same amount of money.

A value-added tax or other sales tax on food and other essentials such as clothing, transport, and residential rents can be regressive.

I suppose it could be considered regressive because if the tax rate is the same for everyone that means people with lower incomes would generally be spending more of their income on goods so they would be paying more of it.