Abby, I wouldn't say that you CAN'T plant them in the same hill. I haven't tried it, and I imagine some would depend on which pumpkins and which melon varieties you tried. Especially with squash, some varieties are A LOT more rampant than others. Baby Bear Pumpkin, for instance, has a very restrained vine. Mine never went much farther than 5' out. But a cushaw will usually produce 30-40' vines. Personally, I'd recommend giving each type of plant some "personal space." But if you experiment, you might want to report back on your results, later in the year.

George
Tahlequah, OK