All-around header dimensions for external wall
It's hard to provide a "straight answer" with the information given. Bearing wall vs. gable wall, rambler vs. two-story. Window position and wall height. There are many factors. I can tell you that triple-member headers are not common except where limited height is available. 

We built nearly all our exterior headers with doubled 2x10s in a U configuration, with insulation between. This leaves no voids when drywalling, hanging window treatments, mounting exterior trim, etc., and we know they're adequate from a structural standpoint. It's only when dealing with spans over 5 feet or so that you start looking to LVL headers.

When in doubt, and when there are no drawbacks other than a little initial cost increase, go with the bigger option.