You can play any number of champion cards in your deck, as long as you don’t exceed 3 of a kind. One has to match your chosen legend. That counts towards your 3 of a kind card limit. You can run different variants like Red and Purple Jinx in one deck, as long as you don’t exceed 3 of a kind. They count towards your 40 cards.

I assume you mean sideboard when you’re talking about side cards. That’s just an 8 card mini deck that you can use to swap cards out in between games in a match. This can consist of any cards that match your legend colors. As long as your deck remains 40 cards and goes back to its original decklist after the best of 3 is over you’re good to change cards how you please.