Hi Maria!
So if you're not wanting to make your checkout page available during those times, you would actually have to manually remove your checkout button code at 3pm, and add it back in at 6pm. Removing which bit of code would depend on the theme, but if you shoot an email to support@shopify.com we'll give you a hand!
If you're looking for more of an automated process, I'm sure this would be possible with the help of a Shopify expert! Check out our top experts page here Maria :).
Cheers, Elliot
support@shopify.com