Technically, she does have only one father.
There's a great debate on whether stepparents should be addressed as "Mom" or "Dad," but it all comes down to personal opinion.
I personally feel that if both parents are active in their child's life, then only the biological parents should be addressed by the title. For example, my sons from a previous marriage simply address my husband as "Brian" even though my two younger, our children together, address him as Dad. We've seen no confusion out of them in doing so.
However, others will argue that being stingy with titles are the result of insecurities. It wasn't for us, but maybe it is for others.
What you need to do is ask yourself this, when your ex remarries are you okay with your daughter addressing her as Mom? Or her being considered as a mother, being given motherly responsiblities for your daughter? If the answer is no, then I think you need to teach your 3 year old the difference between Dad and stepdad.
As for Tennessee law, typically the Courts won't address it one way or another, but such a factor can come into play determining future custody, if it should be argued, as it can be a tell-tale sign of willingness to co-parent, intent of a parent, etc.
Hope this gives you food for thought and helps you make the decision best for daughter.