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

Be disappointed后接的介词常见的有by，with还有in，到底使用哪个是由所disappointed的类型而定。

1.  be disappointed by

Usually indicates that somebody has done something specific to cause you to be disappointed 通常用来表示某人做了一些具体的事情让你感到失望

例如： 

I was really disappointed by Wendy. She asked me to go out with her, but she never showed up! 温蒂实在让我很失望。她要我跟她约会，结果她却没有来！

2. be disappointed with

Implies that the cause of the disappointment was something basic about the nature or attributes of the thing. It 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. 

表示对某个东西的特性、品质感到失望，通常指物，有时候也可以用来指人，但这时是表示对某个人的能力、资格资质感到失望，而不是指人做的具体的某件事。

例如： 

I was disappointed with my new coffee maker. 

我对我的新咖啡机很失望（对咖啡机的品质感到失望）。 

Were you disappointed with the result?

你对结果感到失望吗？

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

我对那个候选人很失望。他不具备我们这项工作所需的能力。

3. be 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. 

 通常表示对于人（或物）的比较深的失望，通常表示对其失去了信心。

I'm very disappointed in Tom. I thought he was capable of doing what I expected. 

我对汤姆很失望。 我本以为他有能力做到我所期望的事情（实际上他没有能力做到）。

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

我对美国政府很失望。他们似乎什么事儿都办不了。