If, as part of a program, I'm sending out three emails over three months (one email per month), is it better to create one project with three parent tasks for the three emails? Or should I create three separate projects within the program? Thanks!

I think it depends on what content feeds your emails.

If these are say pre-planned internal communication reminders of like say "Be sure to update your [health screening biometrics] by [date] to receive your insurance discount!" then I'd just create one project as you already know what the content is going to be.

However, if these emails are external items to clients that don't have pre-approved content and likely depend on other teams to assemble items like email list, graphics, etc.. Then I'd feel OK making those separate projects as you'll be assigning work for them.

That's my though, but one way might fit your org better!