How do you reintroduce meat into your diet after being a vegetarian for 3 years?

Start out with maybe 1 meal a day with a bit of meat. Use white meats; chicken and fish first, and work up to red meats. Just continue to increase the amount of meat over a few months, starting with red meat about half way through.

Also, stay to your usual meals but put meat in, like if you had some protein source in the meal (legumes, tofu, etc.) just replace that with meat rather than changing your entire diet.

Good luck!