Are they all different objects, or is it 20 instances of the same object?
If its all the same object you can do a loop, or repeat 3 times using pick a random instance under sprite actions. However that may pick the same instance more than once. To get around that you'll have to compare a private variable, but on the bright side you can use that variable to tell the system that that particular sprite was picked.
There's a function object you can use for the off chance one instance gets picked more than once... third party.