Which sentence is written correctly? stepping into the mud, my shoes were ruined. the mud ruined my shoes while stepping into it. stepping into the mud ruined my shoes.

Stepping into the mud ruined my shoes.

Explanation:

There's no need for any comma in this sentence. It is a full, complete and ok sentence. The first sentence is broken is two weirdly incomplete sentences. The second is poorly constructed, making it possible to read as if the one doing the stepping was the mud.

to save the day, harry must get past a scary three-headed guard dog.