When an email is responsive, it means that modules will stack when viewed on mobile because one column tends to better fit a smaller screen. If you would like to maintain three columns on mobile, you can make the template a basic template rather than a responsive one by clicking Actions > Make basic. This will be a fixed width email and should display a “zoomed out” version of the email on mobile.
Knowledge base article: https://knowledge.hubspot.com/articles/kcs_article/email/does-hubspot-have-responsive-email-templates