Let's see... on the face of it, there are lots of bands that have "never failed me", in the sense that every album they've produced that I have heard has been great. But if I'm judging them by the criteria specified in the OP, that makes things more complicated.
When you take those criteria into consideration, I'm forced to leave a lot of bands whose virtues I happily extol off of this list because I haven't actually listened to all their albums, and so I'm unable to conclude they have never produced one that isn't good. Other bands I would generally proclaim to be wonderful might have some early material that is less than wonderful, and a whole slew of great bands I would love to mention here haven't actually produced 3 or more albums just yet.
So with that in mind, the list of bands with 3 or more albums that I can personally verify have never sucked is as follows:
Beyond Twilight
Epica
Falconer
Fireflight
Karmakanic
Lullacry
Masterplan
Poets of the Fall
Rhapsody (Of Fire)
Scar Symmetry
Silent Force
Sirenia
Tarot
Tristania