I would first send all three source CSVs to an Aggregator using Group Processing with Group By set to ID. You will also want to set the Accumulation Mode to Merge Incoming Attributes. This will create a single feature for each of the IDs containing the attributes from all the incoming tables. From there you can use an ExpressionEvaluator to perform your operations in the 'Magic Tool', and output to your CSV writer.

Super thanks @dustin​  !! your solution worked perfect for me.

In my Magic tool, I also tried out "AttributeManager" and that seems to also do the work of the "ExpressionEvaluator" is there any pro - con in using either of the two?

No, they can be used interchangeably to get the same result. I prefer the AttributeManager for things this, actually. But I usually recommend the other transformers on the forum just for the ease of explanation.