So we had planned to meet Sunday, and a few hours before we meet, she sends an excuse to cancel (100% verifiable excuse, it's true that she couldn't make it, 100% not lying). After she send a few texts saying sorry, I just responded "K". Left it at that.

When we talk next time, should I be firm and say something like "Next time tell me earlier that you're canceling" to show that I value my time?

Just to make our relationship clear, we've gone on 3 dates, we've slept together on the 3rd date.