The first three groups have a total of 14 + 16 + 12 = <<14+16+12=42>>42 children.
The fourth group has 42/2 = <<42/2=21>>21 children
All together, the camp has 42 + 21 = <<42+21=63>>63 children.
In one day, the children consume 3 x 63 = <<3*63=189>>189 bottles of water.
In 3 days, the children will need 189 x 3 = <<189*3=567>>567 bottles of water.
The camp organizer has already purchased 13 x 24 = <<13*24=312>>312 bottles of water.
The camp organizer must buy another 567 - 312 = <<567-312=255>>255 bottles of water.
#### 255