thank you so much for your replay, and sorry for me being late.
It is interesting to keep this rule of 3 as you say.
Yes, you are right when the USD is second in the currency pair, and when the dollar is strong for example it will force these pairs to go down, and same on the other side. Now since there are only four pairs where the usd is second – EURUSD, GBPUSD, AUDUSD and NZDUSD, I sue it a starting point to figure out the other pairs where the USD is always first.
I realized that when I used the indicator from Petko that shows which is the best currency pair to trade at every moment, it was called FPSM meter I think. Very useful one in manual trading, but now with algorithmic trading I believe it is all about good choice of currency pairs, proper work with EA studio, and good analysis of the generated EAs.