Just curious, was your expectation that she would be home at 3pm based on your own calculations or did she say, "I will be home around 3"?
Based on MY experience:
An event that's scheduled to end at noon, could get out as early as 11am or it might be 1:30pm before you ever get to your car. Add in an hour for lunch and a 2.5 hour car ride (without traffic issues), 3pm seems optimistic at best.
Car trouble: 30-45 minutes just waiting for a tow truck, add in another 30minutes - 2 hours to fix the problem. Flat tire or out of gas would probably only be an hour delay any thing else would likely be longer.
More clarity on time expectations would be a definite MUST. You should not be left to assume or calculate her ETA. You don't really give a shit when the event ends, you want to know when she will be home. So ask what time she expects to be home. If things go wrong, she needs to keep you updated more often and with more specific time expectations. No more "we are going to be home a little later", that is definitely too vague.