Sure, but four or even 5 would be better as long as you keep up with the waterchanges. Might get a little pecking order/breeding aggression as they get older with only three. Keep in mind though -- your end goal should not be to keep only 3 discus longterm...this is not a big enough school for them to feel naturally comfortable. In fact, they look a little unnaturally timid in the video, but they could just be mellow at the moment. Why not throw 2 or 3 more amateurs in there right now, then keep an eye on them for 6 months? They'll be much more happier. You can always remove later to lighten the load or remove overly assertive fish.