It's a custom configuration, so Apple never carry stock of it. Outside of the basic configurations they list, they build it when someone orders it, so some delay is expected. Adorama and B&H are two of the few stores that carry non-standard configurations (which means they're one of the few places to find deals on non-standard configurations too). As they've already ordered these configurations, they have them in stock, while Apple does not.
Edit: depending on if you really need the M3, the M2 MBA/24/2TB is available refurb right now for about the same price, if you'd like double storage more than you need the M3. And those would be in stock, and ready to ship.