That's not an arbitrage. It's assuming that there is no arbitrage To make a profit, one needs to buy low and sell high but this isn't taking the advantage of an arbitrage condition. This is just making a profit through spread. To take the advantage of arbitrage opportunity, one needs to know the intrinsic value of a currency pair
If you do a triangle between USD, EUR, GBP your main risk is that you will slip and get values that don't satisfy the arb condition. You're still completely hedged once the total transaction is complete. In practice, unhedged times are as low as micros as are these arb opportunities. This is why retail can't really trade here.
Sig..why don't you let him do the work? throw up some charts and observe a forth chart of your choosing. I know a trader that made a living off of non-logical discovery (forgot his name)...well I once read you could throw a dart at a stock symbol and make money too (if you traded correctly)...lol haha maybe I wrote that somewhere es