Ahoj,
"Ahoj" is vastly more common than the other two. "Nazdar" comes second and there are many people who have never uttered "servus".
My perceptions:
ahoj - completely neutral
nazdar - sounds a bit military to me, so I avoid it (but it is certainly just me; generally, it is not perceived like that)
servus - somewhat youthful and intellectual flair; you wouldn't greet your grandmother like that
All of them are informal as you probably know.
Jana