Grandma diagnosed with stage 3 non-hodgkins lymphoma. After 3 years of treatment shes completely cancer free. High or low chance cancer will come back?

It depends. There are several different types of Non-Hodgkin Lymphomas(NHL). The common ones include Follicular lymphoma which is low grade and indolent but can relapse even after 3-5 years of remission. The other common one is called Diffuse large B cell(DLBCL). This is potentially curable and very unlikely to relapse after 3 years of remission. So check which su-btype of NHL does she have.
Depends on subtype. There are dozens of different subtypes of non-Hodgkin's lymphomas. Most will remit; curiously, the lymphomas that are naturally more aggressive are more likely to be cured by chemotherapy. Since in your Grandma's case, it's either "cured" or "not cured", why not plan with her for a happy rest-of-her-life, assume a cure, and live richly -- remembering to treasure whatever time you have together.