How can I tally how many text fields are filled in?

I have 3 custom text fields, and one formula field to tally and want to return 1 if 1 of the custom text fields is filled in, return 2, if 2 filled in, return 3 if all 3 are filled in. I tried the ISNULL function which didn't work. Then I saw this in help text which explains why it didn't work:

"Text fields are never null, so using this function with a text field always returns false. For example, the formula field IF(ISNULL(new__c) 1, 0) is always zero regardless of the value in the New field."

Anyone knows a way to make this work? Thanks