Hello! I need to create a workflow to send an email from a template (that I've already created) when a sales order is placed that has three like items purchased of a certain manufacturer and I'm not sure how to go about doing that.

You 1st need saved search. Once you have saved search you’ve two options: 1. Either schedule the email action 2. Or have email button on SO and on click of the button it send email with your template.