Yeah, I would say they are a bit older than 3 months old. It is possibly that they could've started laying early, but the breeds that are bred for laying and meat production generally start laying eggs at 5-6 months old. I have standard pekins though, I do not have experience with the jumbos.
Are you going to use an incubator? Or are you planning on letting the hens set?
From my experience, I've had a 95-100% hatch rate by letting the hens do the hatching for me. That being said, pekins are not known to be broody and set on their eggs. Plus, it's late in the year so I think your chances of having them do it for you would be better in the spring. Right now, the only broody hen I have is a muscovy. All the rest have stopped setting for the year.
With an incubator, I am not the one to ask. I have one hatched a few batches that way, and my hatch rate varied from 60-85%. I did find that you have to keep it moist, turn the eggs by hand not with an automatic turner (I got a WAY better hatch rate turning them 3-4 times a day by hand even though there's less work with the automatic turner). I had to help a lot of them out of the shells too. That being said, there are a lot of people on here that can help you with that. I'm pretty sure there are multiple discussions about hatching duck eggs. You can go to the search button at the top, type in "hatching eggs" or "incubating eggs", and be sure to scroll down to "DUCKS". Otherwise you'll get all of the chicken hatching threads to read through as well.
Good luck and I wish you well!