Your discus are beauties !
How long have you had them ? Not long, I suspect.
A 40 gallon is large enough for 3 discus, but that's not a good number to keep for sociability/compatability reasons. I really hope you don't encounter any serious pecking order/aggression issues.
I suspect you will, somewhere along the line in future, and if you do, I'd suggest you get at least 2 more discus, and a larger tank to keep them in.
Just a caution - don't mean to rain on your parade - your fish and tank are indeed lovely.