You have all the information you need to make a decision. The only person it affects is you, so think about your goals and proceed as you see fit.
Just know 99% of us on this forum would say to keep the card open, do the BT to save yourself money, and be happy you have 3 cards and that you got in with Discover.
I was in a similar situation...a small number of cards but a good CL total and one of those cards was the Amex Plat. But I wanted a new 0% card in order to burn down some of the debt other cards over 18 months. I seriously considered the Discover IT and the Citi Diamond Preferred before coming to the Penfed Platinum Rewards Visa. It would allow the 0%/18 month balance transfer metric I needed and as a bonus, it would be the go to reward card for gas and groceries. Best of all, they asked me during the phone application process, "how much did I want to transfer?". I told them I needed to be considered for a CL of 28k. A day later they asked for income verification (easy) and I was approved two days after that. It was a much better process than playing the lottery over at Discover and not getting a chance of saying how much I truly needed out of the gate.
If you decide to apply for another card, I'd still keep the Discover...nice rewards and shopping choices with that one.