I am using 450g for bandwidth restriction already i have restricted 2 mb on one Ethernet which is in the list of Bandwidth (tx/Rx) and on the same Ethernet i want to restrict to 3 mb but there is no 3mb option available in list give any solution to restrict 3mb on the Ethernet .i have attached jpeg

Hit the wiki manual and start reading about QoS and queues.
Fewi is right, QoS a better solution. You can also type whatever value you want into that field by hand.
I have tried to put manually but the error is coming like this i have tried a lot

This is hardware feature of this board and 3M value cannot be supported.