If deck building is not high on your agenda then you are kinda stuck, without changing your agenda. Despite FFG trying to position this as playable out of the box, and deckbuilding being an advanced game, without it you are going to get very frustrated very quickly. Playing the first two quests with single sphere decks will help you understand the relative strengths and weaknesses of the cards, but the quests are at a level of difficulty that needs a somewhat tuned player deck.
Despite that, even if you don't think you want to do deckbuilding, the simplest form of it is no effort whatsoever. Just pick 2 heroes from one sphere, and one from another, and combine the player cards from both spheres. Then throw away the least interesting player cards (if in doubt throw away the cards from the sphere with just one hero - you will be able to afford fewer of those as you will get less resources) until you have 50 in your deck.
Despite that, even if you don't think you want to do deckbuilding, the simplest form of it is no effort whatsoever. Just pick 2 heroes from one sphere, and one from another, and combine the player cards from both spheres. Then throw away the least interesting player cards (if in doubt throw away the cards from the sphere with just one hero - you will be able to afford fewer of those as you will get less resources) until you have 50 in your deck.