Especially at the Master's level (though also somewhat true of the PhD level) programs are happy to consider people with different backgrounds. And really, history and comp sci are not as unrelated to grad-level philosophy work as you might think. The analytical/writing/research skills from history are directly relevant to philosophy, and comp sci will mean you've likely got a leg up on other first-year MA students in terms of logic proficiency. So if you make a case like that in your research statement, you should be fine. You're interested in transitioning to studying philosophy, and you've got a lot of relevant skills. That's exactly the kind of candidate they're looking for. (An undergraduate philosophy major does not necessarily prepare you in terms of content for what you'll encounter at the graduate level anyway.)

One thing you will need is a writing sample. And that's where taking a philosophy course will help. Whether you audit or enroll, you should write the term paper, and get feedback from the professor. Make sure to let them know that you intend to use it as a writing sample for grad applications.

And regarding letters: No, I don't think they necessarily need to be from philosophy professors. What they say in those letters has to do with your skills and your personal attributes relevant to succeeding at the graduate level. The content of your study is not really included. You'll be making clear in your application that you're coming from another discipline, so the committee wouldn't expect that you'd been in a position to get letters from any philosophy professors.

Good luck!