Dear Atul
Try to understand what Ajay Sir is saying, you don't need to use second costing variant at all. He said that you can use the same (ABC1) costing variant for all plants. Only thing is that you will create procurement alternatives in plant 3 only where you want to use mixed costing.
Using ABC1 costing variant when you will cost the material for plant 3, mixed costing will be done without any problem.
Regards
Rajneesh Saxena