A couple of choices come to mind. Fort Wilderness has cabins that hold approx. 6 people so two cabins is one option. We did that on our first trip and my sister and her family had a cabin directly across from us so it was convenient.
There are new "suites" at one of the All Star resorts and I believe those also hold 6 so if it's possible to get two suites that would work.
DVC is another on-site option for your group. There is another board here on renting points from owners and some posters can give you lots of details on it. Typically, it costs approx. $10 per point and the DVC board has a chart that shows how much each size unit costs during value, peak, holiday seasons. Weekends cost more than weekdays though so keep that in mind.
You could always get 3 rooms at the same resort but I don't know if they can guarentee that you will be close to each other.
I know you said on-site but if you decide to consider off-site check out
www.vrbo.com as one site for beautiful vacation homes. We are renting our third one this trip and we love using them for our vacations. We always rent in developments that are very close to the park so travel time isnt ever an issue.
Good luck with your planning!