I cook is always the present simple not the present progressive.
Actually, if you are in the act of cooking potatoes, you would say "I'm cooking potatoes."
I guess that if you do not know the exact situation, all the three solutions are fine. btw, i'm here to learn.
Cooking, lesson one
Is cozinho always cozinho and never cozinha?
Even though potato is femine?
Cozinho is a verb conjugated following the subject which is "I".