When you decide to trade a Portfolio Expert, you attached in a MetaTrader chart. This makes it possible the included strategies to trade on that chart. If you include strategies calculated for different symbols, they may (most probably) work in the best way on the particular chart.

The best solution is to have three different Portfolios and to trade each of them on teh corresponding chart.

It is possible to merge the different collections you have into one. Use the Validator for that purpose because it allows importing various strategies and collections. Even better, it will push only the validated strategies to the Collection.