Off the top of my head (outfielders by position, although I don't think I'd change if considering three OF):
I'm pretty comfortable with these players. I considered Bench and Piazza for catcher and Ted Williams for left field.
A pitcher causes more hesitation. Candidates: Johnson, Johnson, Grove, Koufax, Gibson, Seaver, Clemens. Choosing four starters is easier than selecting one pitcher for one game. Let's go with:
Four starters chronologically: Walter Johnson, Lefty Grove, Tom Seaver, Randy Johnson. Nice spread in years.
5th starter: Roger Clemens.