How to migrate multiple columns to single field in CSV Migrate
It took a while for me to solve the issue. However, I was able to manage the same without having to write additional plugin or module. I solved the issues in two steps:


First used the plugin concat to concatinate all 3 required fields to one.
Then used the plugin explode to pass the value to the same field.


Yaml Below:

id: sections_tfr
migration_tags:
  - CSV
migration_group: tfr
label: 'Import sections'
source:
  plugin: csv
  path: /var/www/html/php/tfr/prepared/final/sections.csv
  delimiter: ','
  enclosure: '"'
  header_row_count: 1
  keys:
    - s_id
  column_names:
    -
      s_id: Id
    -
      part_no: Part
    -
      sections: Section
    -
      summary_1: Summary
    -
      summary_2: Summary
    -
      summary_3: Summary

process:
  name: sections
  description: heading  
  field_summary:
    -
      plugin: concat
      source:
        - summary_1
        - summary_2
        - summary_3
      delimiter: ','
    -
      plugin: explode
      limit: 100
      delimiter: ','
  weight: s_id
destination:
  plugin: 'entity:taxonomy_term'
  default_bundle: sections
  migration_dependencies: null
migration_dependencies: null