How to not remove but only print multiply rows from CSV file using a awk?

Question

i have the following code

awk -F, '{key=$1 FS $2 FS $3} NR==FNR{a[key]; next} !(key in a)' dubl.txt original.txt

which checking for duplicates in multiply columns and printing only unique lines. But how to print only lines which are only duplicates?