Recommendation letters for assistant professor position (computer science and statistics)
Well, given that you are applying for a professorship position it makes much more sense the second third option. The recruiters do look for external recommendations, but external recommendations in the same area are more valuable as they give more interesting information on the candidate for that role.
However, if i may, i would suggest you to have the first recommendation for your ability to get grants, the second one for your ability to teach and the third one for your ability to research. In this way, the recommendations will be covering all the aspects of the job, without overlapping each other and without overwhelming the recruiters.
I hope my suggestions will help you.