You can do this with some Liquid code, I would probably use variant.inventory_quantity (https://shopify.dev/docs/themes/liquid/reference/objects/variant), to check inventory levels.
Then if quantity < 3, you can either not show the add to cart button, or disable it and add some messaging.
If you prefer to hire out for this feel free to contact my team at speedboostr.com/contact, we do custom coding like this every day.
Another option, if you want to stop selling when out of stock but collect email addresses, you can use the app Now Back in Stock.
Founder of
Speed Boostr (Shopify optimization experts, theme customization, custom app development).
Creator of
Shopify Analyzer (first performance analysis tool for Shopify sites, free for the Shopify community).
Creator of
Order Automator (auto tag orders and customers + auto fulfill orders + email notifications).
Creator of
Theme Scientist (A/B testing app) and
Tip Jar (add a tip button to your store).
