How to grep for the dollar symbol ($)?

% cat temp
$$$ hello1
$$  hello2
    hello3
##  hello4
    hello5 $$$
% cat temp | grep "$$$"
Illegal variable name.
% cat temp | grep "\$\$\$"
Variable name must contain alphanumeric characters.
%

I want to grep for $$$ and I expect the result to be

% cat temp | grep <what should go here?>
$$$ hello1
    hello5 $$$
%

To differentiate, I have marked the prompt as %.

• What is the problem here?
• What should the grep string be?