Index/Rank a Grouped Rdd in Spark Scala

[#1]

I have been really looking to index/rank a grouped rdd. The RDD was grouped by a key as you can see below and I want to index it starting from number 2 for each of the arrays under a key.

What I have

scala.collection.immutable.Map[String,Array[(String, String, String)]] = 
Map( 
        394 -> Array((394,394,0), (394,362,7), (394,368,7)), 
        328 -> Array((328,328,0), (328,324,7), (328,325,7), (328,326,7), (328,327,7), (328,329,7),
        368 -> Array((368,368,0), (368,394,7), (368,396,7), (368,397,7), (368,479896,7)),
        278 -> Array((278,278,0), (278,371,7), (278,372,7))
)

What I want (Notice the the new 4th element of each Array, its an index starting from 2)

        394 -> Array((394,394,0,2), (394,362,7,3), (394,368,7,4)), 
        328 -> Array((328,328,0,2), (328,324,7,3), (328,325,7,4), (328,326,7,5), (328,327,7), (328,329,7,6),
        368 -> Array((368,368,0,2), (368,394,7,3), (368,396,7,4), (368,397,7,5), (368,479896,7,6),
        278 -> Array((278,278,0,2), (278,371,7,3), (278,372,7,4))
  

[#2]

@Joe Widen @Timothy Spann

[#3]

Thank you so much!