Change the wireless channel in router one by one and check performance. However, frequency at which Airtel 3g works is very far away from Wireless channel frequency. It may solve problem. If problem is still persist, observe which devices are on when you use Airtel 3g.
Can u tell me how?and its airtel broadband not 3g and it interferes with my mtnl 3g signal
I know that you are using wifi and You face problem while using both things(Wifi+3g) together. I have a doubt it may interfering each other. That's why I told you to change channel one by one. I 'may' solve the problem.
You can only change channel of WiFi router which is in your control.
Post router details (model No.) if you do not know how to switch channel.