Three Witnesses

From Wikipedia: Three Witnesses is a collection of Nero Wolfe mystery novellas by Rex Stout, published by the Viking Press in 1956 and itself collected in the omnibus volume Royal Flush (Viking 1965). The book contains three stories that first appeared in The American Magazine:

"The Next Witness" (May 1955, as "The Last Witness")
"When a Man Murders" (May 1954)
"Die Like a Dog" (December 1954, as "The Body in the Hall")
Each story in this collection features a witness, not to a murder but to its prologue.