You can use this formula to concat all of the non-blank items into a comma-separated list:
eval(eval((field1 | field2 | field3)[. != ""], 'concat(., ", ")'), "..")
In the bold underlined part, you would list out all of your fields (use the Insert field or group... feature), separated by pipes.
This will append a comma at the end of the list, so if you don't want the final comma, you can use a substring() function afterwards to remove it.