If you could only use 3 metrics to monitor your network, what would you choose?

If you could only use 3 metrics to monitor your network, what would you choose? You can have as many sensors as you want, but you can only choose 3 metrics.

If possible, define those metrics in your own words, and briefly explain why these metrics are the most important.

I want to learn what I should be using for monitoring that perhaps I am not using, and what metrics I should be paying attention to.