What is triangular arbitrage?  What is a condition that will give increase to a triangular arbitrage opportunity?

Answer:  Triangular arbitrage is the method of trading out of the U.S. dollar into a second currency, after that trading it for a third currency that is in turn traded for U.S. dollars.  The aim is to earn an arbitrage profit through trading from the second to the third currency while the direct exchange among the two is not in alignment along with the cross exchange rate.

So many, but not all, currency transactions undergo the dollar. Some certain banks specialize in making a direct market among non-dollar currencies, pricing at a narrower bid-ask spread as compared to the cross-rate spread. However, the implied cross-rate bid-ask quotations impose a discipline on the non-dollar market makers.  If their direct quotes are not constant along with the cross exchange rates, a triangular arbitrage profit is possible.