Your milk is almost certainly not disappearing. It's normal to stop feeling so full at that age, as the hormones settle down.
Things to look for that would let you know there is milk: is he swallowing when on the breast? Does he stay latched on and not fuss? Both of these are indicators that he's getting milk.
Many babies at that age feed more often than every three hours - have you tried offering the breast more regularly? This would help boost your supply if there were issues.
I'm certainly not judging you for mixed feeding, if that's what works for you, then its right. But it sounds like you want to keep BFing and are adding formula only to help with weight/hunger? (feel free to ignore if I've misinterpreted) So I might try to offer a bf more often, to boost milk production, so that as much as possible was breast.
Also, babies who have had a full feed often will still take a bottle afterwards, it's a reflex in response to the teat in their mouth, so doesn't automatically mean hunger. It may be that gradually reducing his formula while offering the breast more could reduce the need to supplement. (again, if you don't want that, that's fine too, but I wanted to offer the idea in case you didn't know you could go back to ebf after mixed)