Answer:
I'm pretty sure the third one is correctly punctuated: Within minutes of hearing the news, we had raced home. Sure enough, our friends had just arrived and were waiting for us.
Step-by-step explanation:
"Within minutes of hearing the news, we had raced home." - This would be the first sentence since you and your friend heard the news and raced home. The other sentence would be "Sure enough, our friends had just arrived and were waiting for us." - This part is after you and your friend heard the news and raced home, so basically it'll become another sentence.