There is actually another level of complexity in the solution as to what are the real Top performers, and this comes from the fact that the pip value for all assets (currency pairs) are not the same. For example, it is recommended that for a 10K account the lot size should be 0.33 lots to limit the risk to 1%. So if you are going to trade the top 3 assets you would think to split the lot size equally between all three assets and make each asset have a lot size of 0.11 lots. However, in reality this does not evenly split the risk as the pip value for each asset is not the same. Another is considering how many pips each asset gained in a given period of time. So there are various ways to determine which assets are the top performers.
Having said this, Petko has kept it simple for new traders by just keeping to lot sizes the same and determining the top performers based upon the profit of each asset over a given time period. It works just as well as any other method!