Is it ok to take miralax (polyethylene glycol) for 3 months?

Depends on reason: No universal answer to this. Chronic constipation can have many causes such as side effect of meds, metabolic disease (eg. Diabetes, hypothyroidism), neurologic disease, low fiber diet, obstruction from colon polyps/colon cancer. This is why over-the-counter laxatives should not be used long-term for symptoms of unknown reason; getting to the root of the problem is highly advisable.

Might be: Agree with dr. Puppala's answer. If potentially serious causes checked for, ruled out, it can be used safely for 3 months. Better to treat the underlying problem, diet changes, etc. Whenever possible.