Catherine drinks 128 fluid ounces of water a day. What is the total volume of water in cups that she drinks in 3 days?

There are 8 fluid ounces in a cup, so Catherine drinks 128/8 = <<128/8=16>>16 cups of water a day.

In three days, Catherine drinks 16 x 3 = <<16*3=48>>48 cups of water. Answer: \boxed{48}.