Hello, I am a regular education teacher in Northwest Ohio. I typically always have students with special needs and they stay in my room for social studies and science. Will you have all of your students until 5th grade? If not, I would suggest tying as much together as possible. Look at the standards and see where they overlap. Third grade social studies covers local government, fourth grade covers state government, and fifth grade covers state government. You could cover all areas of government local, state, and federal at once that way you are meeting the standards for all grade levels. If you will have the students for three years you might decided the years that you will teach the standards. One year you might teach standards for third grade, the next year teach standards for fourth grade, and finally teach the standards for the fifth grade.
I am assuming your autistic children are on alternative assessment. I would create all multiple choice tests. I also suggest lots of hands on projects like drawing and coloring, trioramas are fun as well. Science experiments are good. I have used clay to let students create landforms. I have used sidewalk chalk to let students trace shadows of the sun while studying weather. Anything that you think is fun they will enjoy. Good luck.