Answer: I need to get my crayons from my desk at school.
Step-by-step explanation:
The first option could work, but we would need a comma and the option does not include one. Just as I used a comma in the sentence before! When combining two sentences or two parts with a conjunction you usually will need a comma.
The second option looks good! It can help to read them out loud when deciding which makes the most sense.
The third option only adds a comma, it does not use a word to help the two parts flow. This is called a comma splice.
While this option works, it is not necessarily the best. It breaks the sentence into two parts, but it does not have smooth sentence fluency.