I have flown DUB->US about 6 times.
Short answer is that 3 hours is reasonable. DL will not cut off checkin that early.
Long answer:
. You first have to deal with the check-in process which includes a screening so no zip right through option. As a ST+ w/o bags it takes me about 10 minutes on average, but I guess the masses could take 30 minutes.
. Then standard security, not that bad.
. Then you need to get to US Pre-Clear. This entails another security check and US immegration process.
I usually plan to get to the airport at about T-3;30, but always have plenty of time for a few beers.