Denatured alcohol is not rubbing alcohol.

Denatured alcohol is mostly ethyl alcohol cut with enough methyl alcohol to make it unsafe to drink.

Rubbing alcohol is isopropyl alcohol.

DOT 5 doesn't mix with DOT 3 which is why you want all of the DOT 3 out of there. Use DOT 5.1 instead.

https://www.valvolineglobal.com/en-eur/vehicle-maintenance-brake-fluid/