Recently my wife had a miscarriage; are the conditions of giving birth applicable here and the pregnancy was three months? Can I have sex with her even before she stops bleeding? Or do I wait for it to stop and for how long?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Generally a pregnancy after completing ninety days, i.e. three months takes the form of a human being so, the blood coming after abortion is the blood of "Nifaas" (confinement period). In this case, she should completely avoid having sex, praying and fasting till the blood stops or completes forty days from the day of abortion. For more details, please read the Fatwas 82043 and 82955.
Allah knows best.