In actions (1) affecting the title to real property, (2) for the partition of real estate or (3) for the foreclosure of a mortgage on real estate, if (a) any party having any interest in or lien upon such real estate is unknown to the plaintiff, (b) the residence of such party cannot, with reasonable diligence, be ascertained by him and (c) such facts shall be made to appear by affidavit to the court, judge, clerk of court, master or judge of probate of the county in which the cause is pending, such court, judge, clerk, master or judge of probate shall grant an order that the summons be served on such unknown party by publishing it once a week for three weeks in a newspaper printed in the county where the premises are situated. Such publication shall be equivalent to a personal service on such unknown party.