Presumably the twin engine planes are designed to fly on one engine. And presummably the four engine planes are design to fly on two engines. Three out of four would seem better than one out of two. But this is not the point. Flying on three engines is less safe than flying on four. When you have the whole length of the United States to land for repairs but don't, one has to conclude British Air will take unnecessary risks with passenger's lives. I can easily imagine unfavorable prevailing winds and being forced to fly less efficiently at lower altitudes could have caused this flight more trouble than it did.
But is it less safe than flying on two? Considering that it can actually fly the 60 minute flight path to a divert airfield on just one, flying on three would still leave you two more improbable failures away from declaring an emergency, while flying on two (A310, 777) puts you just one failure away from an emergency.