connecting a LAN printer while using the 3G network
No, this cannot be done. 

The WiFi and the 3G connections can't be brought up at the same time on an iPad. 

You could turn on WiFi when you need to print and get back to 3G after that, or use a service such as Google Cloud Print to print documents from the Internet. 

The Google Cloud Print solution requires a computer connected to the Internet and to the printer.