[1] Their are three erors in this sentence.

The two obvious errors are that "their" should be "there", and "erors" should be "errors". But what about the claim that there are three errors? There are two possibilities:

• If the sentence really does contain only these two errors, then the claim that there are three errors is wrong, and that counts as a third error. Simple, right?
• Not so fast! If there are really three errors ("their", "erors", and the claim that there are three errors), then the statement that there is a third error is correct.

We have a paradox: are there really two errors, or are there three? If you don't want your readers to work this hard to figure out your logic, you need an editor's help. The simplest solution involves a rewrite: "The phrase 'their are three erors' contains two errors."

The more important point is that if you focus on obvious errors, such as the two typos, there's a risk that you'll miss subtler logical errors that may prove to be far more significant to your readers.