Credit scores are partially determined by how much total credit you have available, versus how much credit you have charged (how much you owe). Better to owe $1000 on a $5000 total available credit, than to owe $1000 on a $3000 available credit (for instance). Even looks better to owe nothing on $10,000 available credit, than owe nothing on $8000 (make sense?).
Since those cards are currently at zero balance, even if they have a low available credit, they make your credit look better.
What I might suggest is to get the Discover card, THEN cancel out those two other cards. That way, you qualify for the best deal from Discover, BEFORE you ding your score a little bit.
And while cancelling a credit card that you've had for a long time can also ding your score, if you have another one that's nearly as old an account, it really won't ding you too badly. Back when I first tried re-establishing credit, I applied for a card that was easy to get, but hit me up for $45/year. I cancelled it, last year. So far as I can tell, it did NOTHING to my score, since I had another card that was nearly as old of an account (13 years old, versus 14 years for the cancelled card).