Once I said" Wallah" (By Allah) this house belongs to my friend and actually it was my uncle's house. I told so knowing that it is my uncle's house. But the word "Wallah" came out by mistake then I asked forgiveness of Allah. Now what should I do? Do I have to fast 3 days or not.?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The oath you mentioned in your question is an unintentional oath, therefore, you do not have to pay any expiation for it. Allah Says (interpretation of meaning): {Allâh will not call you to account for that which is unintentional in your oaths, but He will call you to account for that which your hearts have earned. …}[2:225].
Allah knows best.