Try adding a video to the thank you page that builds trust between the registrant and the presenter. Give them next steps to take. Build excitement. Mention some free gift or workbook you will be giving away to the when they show up live. Have your emails leading up to the webinar build anticipation for the content and how it is going to solve their problem. Talk about students who have transformed after attending your webinar. 3 weeks is a long ways out for a webinar. Typically 4-7 days is a good range. People forget that long out. Give them a reminder in your thank you video to add to their calendar. Have plenty more tips. Hope this helps. Reach out if you need further assistance.