Heat 2 tablespoons oil in a skillet over medium heat. Add rice; cook and stir until browned, 2 to 3 minutes.
Heat the remaining 1 tablespoon oil in a pot over medium heat. Add onion; cook and stir until tender, about 5 minutes. Add meat and sazon seasoning; cook and stir until beef is hot and slightly pink in the center, 7 to 10 minutes. Add the cooked rice, tomatoes, and water. Bring to a boil. Reduce heat to low. Cover and simmer, stirring occasionally, until rice is tender, 18 to 24 minutes.