Can I have 3 cups of diluted coffee in the morning? Is that going to affect my insulin?

It's not going to affect your insulin. If you can handle 3 cups of coffee and if you sleep okay should be fine because the biggest problem with the caffeine is if your liver is really strong and it can detoxify the caffeine and you can sleep go for it. But if you start having it affect your sleep, then we know that's the issue.