Given that merchant fees are typically lower than 3%, and that there's also scheme fees, cost of capital (in a post-ZIRP environment), operational, and fraud costs to be paid from that, (much) less than 3% actually ends up with the issuer (which they can then share with Robinhood as the program manager).

In fact, interchange fees are actually public, so you could even look them up and do the math:

https://usa.visa.com/content/dam/VCOM/download/merchants/vis...

https://www.mastercard.us/content/dam/public/mastercardcom/n...