Well, there are three “romantic comedies” in which Day, Hudson and Randall starred together:
Pillow Talk (1959)
Lover Come Back (1961)
and Send Me No Flowers (1964).
(Yes, I know - it feels like there should be at least half a dozen of them, but there you are.)
Lover Come Back fits your recollection best, given that you are sure it was Doris Day and you aren’t confusing Rock Hudson with David Niven, James Garner, Clark Gable, Cary Grant, Jack Lemmon or Rod Taylor (never mind, the plots don’t really match).
One slight possibility would be Caprice (1967) in which Richard Harris uses a truth serum on the Doris Day character.
Tangent: Tony Randall did make a film with Debbie Reynolds, The Mating Game (1959), playing an IRS man.
Fun fact: Doris Day also made a film with Ronald Reagan (sans Bonzo), The Winning Team (1952).
… or:
You confused Rock Hudson with Richard Widmark and Tony Randall with Gig Young and mean
which has two cocktails followed by a tranquilizer as a pivotal plot point.
