How to join several ArrayBuffer mutable in scala?

Question

I have three 3 csv files. I put them in their ArrayBuffer collection.

How can I join one collection to another, and then the third file with this last one created?

  val file_1_csv = Source.fromFile("./inputs/file_1.csv")
  val file_2_csv = Source.fromFile("./inputs/file_2.csv")
  val file_3_csv = Source.fromFile("./inputs/file_3.csv")

  var collection_file1: ArrayBuffer[Model1] = ArrayBuffer()
  var collection_file2: ArrayBuffer[Model2] = ArrayBuffer()
  var collection_file3: ArrayBuffer[Model3] = ArrayBuffer()

  var collection4: ArrayBuffer[Model4] = ArrayBuffer()

with models :

Model1(
  "FK_ID"   -> String,
  "key1_f1" -> String,
  "key2_f1" -> String,
  "key3_f1" -> String,
)

Model2(
  "FK_ID"     -> String,
  "FK_NUMBER" -> Int,
  "key1_f2"   -> String,
  "key2_f2"   -> String,
  "key3_f2"   -> String,
)

Model3(
  "FK_ID"     -> String,
  "FK_NUMBER" -> Int,
  "key1_f3"   -> String,
  "key2_f3"   -> String,
  "key3_f3"   -> String,
)

Model4(
  "FK_ID"     -> String,  => common for all models
  "FK_NUMBER" -> Int,     => common for all models
  "key1_f1"   -> String,  => provide Model1
  "key2_f2"   -> String,  => provide Model2
  "key3_f3"   -> String,  => provide Model3
)

I use "for" to fill the collections of csv files

Return :

Model1("fkid", "fknumber", "value1", "value2", "value3")
Model2("fkid", "fknumber", "value4", "value5", "value6")
Model3("fkid", "fknumber", "value7", "value8", "value9")

Consolidate Collection4 information following rules::

Join file_1.FK_ID to file_2.FK_ID and to file_3.FK_ID => ID Join file_1.FK_NUMBER to file_2.FK_NUMBER and to file_3.FK_NUMBER => NUMBER

I must have a single collection which is the join of the 3 files.

I would like to get this result after the joins:

Model4("fkid", "fknumber", "value1", "value5", "value9")

the join is made with FK_ID and FK_NUMBER

Thanks for your answers.