You need to realize that the default bin you specified with coverpoint A does not count towards the overall coverage results.
Since you have created individual bins for each explicit value in coverpoint's A1 and A2, the only way to do this is by explicitly list the remaining 11 values for coverpoint A3.
To do this programmatically, you can but the bin values for A1 and A2 into an array, and then use an expression for coverpoint A3.