If I found out the child is mine and two other guys had already been tested why do I have to pay backpay could I give up all rights

No you can't give up rights. There is a myth that a parent can just give up parental rights, but its only a myth.

Whether the mother will be entitled to retroactive support depends on local laws and other circumstances. Normally retroactive support is not granted. Who is saying you have to pay? And why wouldn't you want to provide for your child?