The Three Month Period for a Wife before Re-Marriage
Source for the fact that a widow is permitted to marry other men: Kiddushin 13b
Requiring a three-month gap after widowing or divorce: Eruvin 47a
At what point the three months' count begins: Gittin 18a
Requiring a three month gap even if the wife could not have been impregnated shortly before the death or divorce: Eruvin 47a
Requiring a three month wait even if the husband and wife had only been engaged: Eruvin 47a
Levirate Marriage is done for a wife who a man divorces and then remarries [in a permitted fashion]: Yevamot 108b-109a, 109a
Above, if she was a Minor eligible to Refuse the Marriage at the time of the divorce: Yevamot 109a
