If you're talking strictly three notes per string as per you question, then only one answer is alternate picking, the other is economy picking (and the other is nose picking!). Any time you change strings and keep the picking direction the same (down down while ascending, up up while descending) then it's economy picking.
And, IMO, there IS an absolute answer to this question. I too a few years ago changed my picking from strictly alternate where ever possible to economy picking where possible. At first I thought it was great, more fluid, faster. But over time things changed, and, here is my answer - it's all about how it sounds. Economy picking doesn't sound the same as alternate picking, nor does it sound like all down strokes. IMO it sounds weaker and I don't like as much. I still use it, sometimes you have to. Sometimes it's simply a matter of making the beat stronger. For example, a lot of the time I'm playing three notes per string but the beat is in four so I want a heavier down stroke to match. Most of the time if it's just scale patterns it doesn't matter that much. Sometimes I'll track a riff all down strokes just to get that sound even though I'd probably never play it that way.