HotS and Overwatch really aren't exploitative sure, but Hearthstone is incredibly exploitative. Unless you are so good you can go "infinite" in arena (always make your gold entry fee back and then some, on average), you have to buy packs for every expansion that comes out. Or you have to play with lower-tier cards, which some people do successfully, but to get the most out of the game or be "competitive" they will need the best cards eventually.
Out of the 3 games, I probably played Overwatch the most, followed closely by Hearthstone, but the amount of money I invested in Hearthstone is easily 3-4x the initial price I paid for OW. I got fed up of the "pay or fall behind" nature of Hearthstone and stopped playing a long time ago though.