I am taking 800mg of skelaxin (metaxalone) 3 x daily and still having bad muscle spasms in my back. Can I take 1600mg at one time and would that help?

Check with doctor. You should never change dose of a medication (especially things such as muscle relaxants) without checking with your doctor. Certainly taking DOUBLE the dose all in 1 fell swoop as you suggest is out of the question and I'm sure your medication papers will tell you this. You may need to be re-examined to see if you're really suffering from muscle spasms and what their cause may be for better TX.