The
Two Instrument Stochastic, my first guess, would be best used with two pairs, but only three currencies. ie EURJPY EURUSD. Thus you could see, without looking at a separate currency strength/correlation chart (
viewtopic.php?f=17&t=987&hilit=currency+correlation) (other form
viewtopic.php?f=30&t=3093#p7209), whether or not the EUR is strong when both pairs rise or that JPY and USD both are weak. This is an abstract correlation instrument so to speak. It doesn't show price, it shows their Stochastic.
Technically, look for divergences, oversold and overbought.