Issues when modelling the modern age foreign exchange market?

When we model a two-party system, the model is extremely simple, once a piece of information, which is applied to either or both parities, is released, then there will be a response in the foreign exchange market that one currency's price increases and the other' price drops accordingly. However, in a model with three participants, when one factor changed in one participant has different impacts on the other two parties, then the ratio of the values of the two parties changes without any factor change directly happen to them. Moreover, when the number of the participants increases, the complexity of the model increases, and it is more difficult to estimate the true value of one party, especially when there is a possibility for any other party's to be mis-estimated. Therefore, any outcome should not be considered as the true value.

In order to simplify the model, I may assume that we only test one country's currency and ignore the possibility of other currencies' being mis-estimated. Moreover, we need to find a state that all currencies represent their values respectfully. The time just before one country abolished the gold standard may be able to be used as a state that truly represent one country's currency price.  In addition, as some countries have never adapted to the gold standard, so how do we find a point that we agree that the currency is fairly valued. There is another question that how we judge if the impact is an overreaction that leads to mis-estimation, what is the standard for overreaction. These are the issues I can think of for now when modelling the modern age foreign exchange market.