Struggling when combining 3 variables

I'm trying to combined 3 variables, they are all links that need to be merged into one variable to use as multiple attachments,

{{Letter}} {{SubmittedDocuments1}} {{SubmittedDocuments2}}

The Letter URL always has a comma after it so goes nicely onto either or Docs1 or Docs2, however when trying to combine Docs1 and Docs2 I'm struggling as sometimes they are individual links, sometimes they have multiple links split by commas, however I need to some how add a comma to the end of the variable of SubmittedDocuments1

I tried {{Letter}} {{SubmittedDocuments1}}, {{SubmittedDocuments2}}

Do I need to parse them first or something?