Is it disappointed with, in, or by?

The difference between “in”, “by”, and “with” in this context actually depends on the nature or type of disappointment you feel:

“Disappointed with” implies that the cause of the disappointment was something basic about the nature or attributes of the thing:

I was disappointed with my new toaster. It really didn’t toast the bread as well as I hoped.

The “with” form is usually used with inanimate objects (like toasters), though can sometimes be used for people if you’re expressing a general disappointment about their qualifications/abilities/etc, rather than being disappointed by any specific thing that they’ve done:

I was disappointed with the second candidate. He didn’t really have the skills we need for the job.

On the other hand, “disappointed by” usually indicates that somebody has done something specific to cause you to be disappointed:

I was disappointed by Fred. He said he’d give me a ride, but he never showed up!

(The “by” form is occasionally used with objects, but most of the time only makes sense for people.)

“Disappointed in” usually indicates a deeper level of disappointment with the nature of somebody or something, or repeated problems with them, and often indicates that the speaker has lost faith in someone’s ability to do what’s expected of them:

I’m very disappointed in Bill. I thought he had experience with this job, but every time he does it he does something wrong.

or

I’m disappointed in the government. They just can’t seem to get anything done!