I doubt that it would hurt her. I have friend that is on 4 liters ocassionally, but 2 is more standard.
What is your wife's diagnosis? Does she have chronic lung disease (COPD)? If so the usual upward limit for O2 flow for someone with COPD is usually 4 liters with nasal cannula. So, I think the 3 liter flow would be okay. Do her saturations drop when she walks or excerts herself? You can also try getting her sats up by having her move around more, and take frequent deep breaths and cough hard. Did they send her home with an incentive spirometer? If so, have her use it frequently and if she smokes...........stop!