If my employer gave all employees a Christmas bonus except for myself and2 other people, is this discrimination?

It depends on just why you weren't paid a bonus. If it was due to the fact that your employer feels that you didn't deserve one because your work product was less than satisfactory, not paying you a bonus is perfectly legal. This is true whether or not other employees receive one. The fact is that employees need not be treated equally (or even fairly). That is so long as no actionable discrimination is the reason for the differing treatment. In other words, it you were not paid a bonus because of your membership in a protected class, than that constitues actionable discrimination, and as is illegal. So if you weren't given a bonus because you were a woman, or a Muslim, or disabled that would be against the law (legally protected classes are those based on gender, religion, disability, race, etc).

Additionally, not paying you a bonus is unlawful if failing to give one violates company policy or the terms of an employment contract or union agreement.

Otherwise, as a general rule an employer can set the conditions of employment as it sees fit. In turn, an employee can choose to work for as particular employer or not. This is called "at will" employment.