Currency triplets - arbitrage opportunity?
Think currency rate as simply the price, on the currencies market, of one currency in terms of another. But this is not a perfect market, so to answer your question with some concrete cases:


even if you want to purchase at time T(price) a certain amount of currency at price P, it's not guaranteed that at the time T(transaction) the price is still P; market is open 24/7 with several transactions every minute that modify the price with rapid changes in supply/demand;
currency market is not like a grocery store where there are two actors, a price, a transaction, and calculations can be done on the back of the envelope. To sell / buy currencies you need to involve external agents like banks and third-party agencies, all contributing to the final real price;
risk is not 0: as per first bullet, events can cause currencies fall several points, and the losses should be considered in the real-rates model.