John Gill Commentary 1 Samuel 19:21

"And when it was told Saul, he sent other messengers, and they also prophesied. And Saul sent messengers again the third time, and they also prophesied." — 1 Samuel 19:21 (ASV)

And when it was told Saul
That the messengers he had sent, instead of seizing on David, were prophesying of him, or however were attending to services of a different nature than what they were sent upon:

he sent other messengers, and they prophesied likewise ;
when they came to the same place:

and Saul sent messengers again a third time, and they prophesied also ;
joined the rest in singing praises, or foretelling future events.