I usually assume a gallon of water weighs about 9 pounds.
3,100 gallons at about 9 pounds is about 27,900 pounds
The pool is a circle with diameter 15 feet. So its floor area is Pi*7.5^2, or about 176 square feet.
So, the weight of the pool is about 27,900 / 176 = 158 pounds per square foot.
I vaguely recall that most residential floors are designed to hold around 50 pounds per square foot. The pool exceeded that by a factor of 3.
You got lucky. :-)