If I have white hearts, soul hearts, and normal hearts, in what order will they be taken when I am damaged?
The normal priority excluding the blood machine is soul hearts, then white hearts, then regular hearts.
If you use the blood machine at more than half a regular heart, the priority becomes white hearts, then regular hearts, then soul hearts. At half a heart, it will consume white hearts, then soul hearts, then your last half a heart (killing you). This means if you have a white heart, it will be consumed first on a blood machine - watch out!
It depends on the source of damage.
Generally, from monster hits, spikes and others, the order is soul -> white -> normal.
The blood donation machines are an exception because they will always drain your white hearts first then your normal hearts, until you have half a hart remaining, THEN drain your soul hearts