Do ASTs after 3 years become protected tennancies?

Tenancy Types and Management

I have just been warned of the following: "If you let your tenancy agreements roll over and become periodic tenancy agreements and do not renew the AST before the 3rd anniversary of the same tenant being in the property, the AST automatically becomes a protected tenancy due to the length of time they've been on the same AST". Is this correct? Interested as I have a number of tennancies that will reach 3 year periods later this year.