Can celexa (citalopram) show side effects after 3 days?

Certainly: Yes, it can show side defects after the first dose. Don't get too excited about the side effects unless hey are severe. Many of the side effects subside after a week or two. Consult you doctor for more recommendations.

Yes: Unfortunately the side effects are usually at their worst in the first week, and will get better with time. Talk to your doctor about strategies to deal with the particular side effects you are having.