DR with multiple components

How does DR with multiple components work?

Take a vampire for example. DR 10/Magic and Silver.

A +3 weapon overcomes DR/Silver. Does this mean that a +3 weapon overcomes both the magic and silver component of the DR? If this is the case, why do creatures with epic DR have any other DR types? Pazuzu has DR /Epic, Silver and Good, but an Epic weapon would already overcome both DR/Silver and DR/Good, so why even add them? If this is just for the edge cases where an Epic weapon has a low base plus, than why do artifacts like St. Cubert’s Mace specifically say they act as cold iron/silver?

Does the weapon have to ‘separately’ overcome each resistance? In this case, one ‘plus’ of the weapon would be used to overcome the magic resistance, and the remaining +2 would be insufficient to over come the DR/Silver.

Perhaps the plusses can’t overcome it at all? +3 weapons only overcome DR/Silver alone, but even a +5 non-silver weapon can’t overcome it because there’s no provision for it to overcome DR/Magic and Silver.

Please, this is killing me.