"are" is correct because the subject of the sentence (we three) is plural.

I would say "us three are a team."

Thanks, Kyle. This helps.

An easy way to remember whether to use "are" or "is": If the subject (noun) is plural, then you would use "are". If the subject (noun) is singular, then you would use "is". In this case "we" is plural, so you would use "are". However, if you are referring to a collection of people, the collective is an exception. Example: "The group is on their way to the park." In this example "group" represents many people, but the group as a whole is a collective. There is just 1 group. Another example: "The team is winning the game." The "team" is a collection of people, and it represents 1 team. The English language is full of exceptions for rules, but I believe this is the case more oft than not. Hope this helped! -Kyle