[stinkypete]

this kind of trading is known as arbitrage. the same concept applies to financial securities in many other markets too, not just currencies.

the profits to be made in currency arbitrage are tiny, if they even exist at all. the bid/ask spreads will be such that going A->B->C->A will almost always cost you money. if an opportunity to profit from such a trade somehow comes up, someone's computer machine will pick up on it and make the opportunity disappear by buying and selling to bring the prices back into sync almost immediately.