thermodynamics - Degrees of Freedom of a Linear Triatomic Molecule

I was introduced to a formula for finding the DOF of a molecule which was 3N-k and i was told, it was just for translation and rotational degree of freedom. Here N is the no. Of atoms in that molecule, and k is the no. Of independent Criteria(like Bonds). For example, if its a diatomic molecule, then DOF=3(2)-1 which is 5. But by this, when i find for Triatomic Linear molecule DOF=3(3)-2 which is 7. And Visualising the molecule i could find only 5 DOF. So I raised up the question, but I didn't find a satisfactory response. If there are any considerations or something thats missing, please help.