Is there any truth to this rumor that if you make three Shidduchim you secure a place for yourself in Gan Eden? Where does this rumor come from if there is no truth to this at all?
I don’t know of any source which indicates this.
Certainly, it is a great Mitzvah to get involved in doing Hashem’s work to bring about Shidduchim. The reward for doing this is great. Many special things are written about someone who does Shidduchim for the sake of Heaven without intending to receive money – good kids, atonement of sins etc.
ראה כרם שלמה סימן נ’ ס”ק י”א. שלחן העזר סימן ג’ אות א. נעימת החיים ע’ נט.
See here as well.