The sentence that uses correct subject-verb agreement is:
The hard tests come at the end of the fall.
The other two sentences have subject-verb agreement errors:
- "Both of my dogs has collars on them." should be "Both of my dogs have collars on them."
- "Either the girls or the boy walk in the evening." should be "Either the girls or the boy walks in the evening." (The verb must agree with the closer subject, "boy," which is singular.)