skeletal system and the muscular system,
Muscles are attached to the skeleton and work assuming that the skeleton is there; for example, there is no muscle that makes sure you don't bend the middle of your arm, as it's stable because there is a bone in it. Basically, the skeleton is a scaffolding for the muscles.
the skeletal system and the nervous system,
Many nerves run along the skeleton, or even inside the bones, and the bones protect the nerves; for example in the spinal cord
the muscular system and the nervous system.
The Nervous system controls the muscles; it sends the muscles commands
