If this isn’t your first role, just think back to the first three months of your previous job and remember what you did and what you would try to improve on this time.
“My primary goal in the first 90 days would be to immerse myself in the company’s design processes, user research methods, and design systems. I would likely do this by conducting a comprehensive UX evaluation, so I can both learn about existing systems and begin to pinpoint areas for improvement at the same time.”