For most Data Sources, it is not possible to create a custom metric with Metric Builder with more than one dimension selected (Google Metric Builders are exceptions as they typically allow multiple dimensions to be selected).
However, there are ways you could easily report on the variety of dimensional metrics you want, such as creating separate custom metrics for each dimension.
Once you have your first custom metric created, simply duplicate the original custom metric and update the dimension selection. Click Save and repeat for any other dimensions you'd like to report on.
You can visualize these metrics in multiple tables on your dashboards and reports.