One pumpkin vine will astonish you. And if you plant only one, it will do better not having to compete with another close-by plant. You would likely get better fruit, and almost as much of it, by planting only one.
I am concerned about the 8" depth however. Pumpkins put down a water-seeking tap root system. Why not just forget the pool, cultivate and prepare a sunny spot, and grow your vine there.
If the pool is being placed on a non-cultivatable spot, then I understand. In which case I really would advise only a single plant.

Yes you can grow 3 plants in there but as they say they like a deep root system so first drill alot of hole in the bottom of the pool and put it on the place you want to grow it but first somewhat loosen the soil before you place the pool on top of the place that will encourage the roots to go thru the holes at the bottom of the pool then this is the critical part do not put any gravel or stone in the bottom of the pool but fill it with 1/4 bagged composted manure+1/4 spagnumn peat,+1/4 of your own compost or some great garden type soil ,+ 1/4 or a little bit more of perlite which is good for drainage and air space for good healthy root growth add about 1/2 to cup of lime,about a quart of granite or basalt dust or sand from a rock quary pretty cheap about 2 dollars for 5 gallons they have a huge pile of it from crushing and snapping there stones its in there way a couple of cups of organic time release fertilizer 3/4/5 or 4/4/4 or 3/4/6 ,blood meal if you have it they love nitrigen,fish bone meal is great it has lots of phosphorous to get it off to a great start and sets the blossoms mix all these ingredients together throughly and put in the pool and you will have fantastic pumpkins but be sure to put alot of hole hopefully all at least 1/2 in bottom of pool set on some loose soil with somewhat good drainage and plant them and eventually the roots will work there way thru the big hole into the soil below the pool great idea try put a little fertilizer under the pool before you put it down and fill it with soil good luck I think you will have success