If you have a differential amplifer, you can set Rf/Ri to be 1.5, because the effective gain will be double. If you want a single ended gain to be 3, you can set the Rf/Ri to be 3. Choosing the values of the resistors depends on how you want your closed loop frequency response. But if you are integrating the resistors on the chip(in your design), they better be around 50K -100K so that the DC gain is also not hit and the area occupied is also reasonable