use an eval to concatenate the fields, and in the case a field is missing, use a sed command to remove the extra separation commas.
mysearch | fillnull value="" field1 field2 field3 | eval myfield=field1.",".field2.",".field3 | rex field=myfield mode=sed "s/([,]*)}/,/g" | table myfield
remark : sed command not tested, please try or replace by some logic in the eval.
Thats what one would like to think, but if either of the fields is missing altogether, the eval fails.
You'd have to use
fillnull or similar first.