Hi Horse3farm,
Forms across variants always cause issues. All the code for the site is served on loading. When you alter the browser the script id's are not updated.
Try placing the form in a seperate web folder and load that page within a placeholder.

EDIT: I don't think this will work in this case as it's a log-in form presumably required to enter the site. Perhaps you could create three identical but seperate forms with different ID's? placed within the 3 variants?