How do the Blutsauger's and the Amputator's passive health regeneration modifiers stack?

The amount of health a Medic passively regenerates is also affected by how recently he was damaged. The +3 health per second is the "recently damaged" healing rate.

Over the next 10 seconds of taking no damage, the Medic's self-heal rate increases until it hits +6 health per second. Note: Fire and Bleed are damage, so don't think this will save you from those two statuses.

Incidentally, this increased healing rate after not taking damage also applies to being healed by a Medigun... and to all 9 classes in this respect. This is why sometimes you seem to heal classes faster than normal. According to the wiki, this goes from normal to triple healing rate between 10 and 15 seconds out of the patient being out of combat.