Where can I find the WP3 form for a Thai work permit?

Application for WP has to be done in person, you do not have staff to pick up a new form at Ministry of Labor and fill it in for you. Must be in Thai language anyway.

Good, apply for the work permit for your new employee. When accepted you get the form needed for your employee to apply for a Visa.

in short: The wp3 needed for Visa you can only get from Ministry of Labor after application of Work Permit by Employer.

The WP3 form is to be used by the Employer, to apply for a Work Permit, not for an Employee to apply for a Visa. When the employer hands over all the documents to apply for the work permit, he will get a WP3 in progress form and that one is needed to apply for the Visa application.