Final answer:
The 3rd firearms safety rule is to keep your finger straight along the frame until you are on target and ready to fire.
Step-by-step explanation:
The third firearms safety rule is B. Keep your finger straight along the frame until you are on target and have decided to fire. This is a fundamental part of firearms safety, emphasizing the responsibility of the handler to prevent any accidental discharge by ensuring that the trigger is not inadvertently engaged. Understanding and following this rule, as well as other firearms safety guidelines, is essential for minimizing risks when handling weapons.