Col_A | Col_B
--------------
  A    |    1,6,5,3,7
  B    |   17,18,78
  C    |    90
  D    |    56,57

Col_A  |  Col_B
----------------
A      |  1
A      |  6
A      |  5
B      |  17
B      |  18
B      |  78
C      |   90
D      |   56
D      |   57

val origDF = Seq(
  ("A", "1,6,5,3,7"),
  ("B", "17,18,78"),
  ("C", "90"),
  ("D", "56,57")
).toDF("Col_A", "Col_B")

val valueAsArray = udf((value: String) => {
  value.split(",").map(_.trim).take(3)
})

origDF.withColumn("Col_B", explode(valueAsArray($"Col_B"))).show(false)

+-----+-----+
|Col_A|Col_B|
+-----+-----+
|A    |1    |
|A    |6    |
|A    |5    |
|B    |17   |
|B    |18   |
|B    |78   |
|C    |90   |
|D    |56   |
|D    |57   |
+-----+-----+