I don't believe there is an answer to black-red-white. Esper refers to one of the Shards of Alara, of which there are five. From a post on TappedOut and MTGSlavation:
Bant - primary white, secondary blue/green
Esper - primary blue, secondary white/black
Grixis - primary black, secondary blue/red
Jund - primary red, secondary black/green
Naya - primary green, secondary red/white
