the best card to buy is eth $400 and you will get your roi in few days, there is no better card to buy right now. With $3k you can buy 7 eth cards.
Do you mean just straight buy eth i am assuming? If so roi is instantaneous
OP If you are new to the game i would spend a lot less, learn the ropes and buy straight crypto with the rest. Once you get your feet wet and learn your way around then go in deeper. My 2 cents