I haven't been in the 3 bedroom units at Kings' Land, only the 1 bdrm and 2 bdrm. Have you checked the floor plans to identify the larger units in each category? I know that the 1 and 2 bdrm. units have two different sizes. The ones in Phase I are larger than in phase 2 and 3, but also require more points. If the same applies to the 3 bdrm. then that might swing my vote to the 3 bdrm., so you have one larger gathering area, but with all adults and 3 couples, the advantage of going with the two bedroom units is that each group has its own space.
Whatever you choose, you I am sure you will enjoy Kings' Land. It is a great resort, plus privileges at the Hilton Hotel complex for a change of pace.