In the case of sponsors who are not baptized how can we determine, whether or not the sponsor selected have already received the three sacraments - baptism, confirmation, marriage?

In case of doubt, the secretaries should to make sure that the parish priest performs an investigation.

In requesting for a baptismal certificate, why is it necessary to indicate the phrase “For Marriage Purposes” on the document?

The “For Marriage Purposes” remark on the baptismal certificate serves to indicate the change of civil status of the individual. It also allows the parish to easily identify if the person has been married in the Catholic Church.