I see that on many icons the Mother of God has 3 eight-pointed stars on her clothes (forehead and shoudlers). What does it mean?

Among other things: There are seven days of Creation. The eighth day is therefore the beginning of the new creation.

In the "Week of weeks" spanning from Pascha to Pentecost (with each Sunday corresponding to the corresponding day of a seven day week) Pascha is literally the beginning of a "week of Sundays"; Pentecost is the beginning of the new creation, the Church.

Beginning of the new creation...play that along the Troparion of the Annunciaiton "Today is the beginning of our salvation....".

St. Paul declares that "anyone in Christ is a new creation"....surely she who bore the Incarnate Word should be the first to be thus renewed in her Son!