Create a new linked record in Table B (junction table)
Search for a record in table C bases on a field in Table A
Enter a value from the record found in Table C to the record created in Table B (step 2)
I’ve experimented enough to know Step 2 can be done with an Automation, but I suspect the whole workflow will require scripting. Something like this would be fairly easy in FileMaker Pro and I’m hoping the same is true for Airtable.
Unfortunately, the vast majority of what is easy & trivial in FileMaker Pro is not possible at all in Airtable… at least not yet.
It currently is not possible to use a value from a record identified by the “find record” action.
You are correct that this workflow could be accomplished fairly easily with scripting. You could use run the script either from an automation or from a button. If you are not comfortable with scripting, there are several professional script writers on these forums, including myself.
By the way, is step 1 of your workflow done manually or do you want the record to be created automatically? If it is created automatically, how do you get the field value used in the search in step 3?