I'm confused. Do you mean you fast for 12 hours on each of Monday, Wednesday and Friday, totalling 36 hours? Or you fast for 36 hours starting on Monday, Wednesday and Friday... which would be all of Monday and half of Tuesday etc?
If you sleep for 8 hours and don't snack before bed, you would probably "fast" for 12 hours in a day incidentally
Which is why that next meal is called "break fast".
> 36 hours, 3 times per week