Three is/are enough?

Both are correct. Asking questions can help to clarify what is happening:

• Three aren't enough. There aren't enough what? There aren't enough dogs.
• Three isn't enough. What isn't enough? Having only three dogs isn't enough.

In the first case, three (dogs) is the subject of TO BE, hence the plural. In the second case, (having only) three (dogs) is the subject of TO BE, hence the singular. My suspicion is that which case is more common will depend on the variety of English, as well as the noun in question. Nevertheless, both are definitely in usage, as can be illustrated with another example, where all the following are in usage:

There aren't enough spoons. Are there enough spoons? Are 3 spoons enough? Is this enough spoons? Is 3 spoons enough?