So, if I draw 4 cards, I lose 0+1+2+3 = 6 life? Not 3+3+3+3 = 12 life?

I believe so.

The for-each-card part isn't a compile-time check, so to speak, so it would be 3+3+3+3.

But really, it works as "If you would draw a card, instead draw your deck and win in response to all the life loss triggers. Or draw more than that because of Laboratory Maniac, and the triggers never even touch the stack."

Any way to fix it?

Probably should limit the number of cards you can draw. Or at least change it to "Whenever you would draw a card, you may pay 1 life. If you do, draw an additional card. Repeat this process any number of times." Then at least you lose life before drawing cards.

Hmm... my whole inspiration for this was having it say "draw any number of cards."