MLIS churns out lots of videos -- any chance you could narrow it down by providing a link to the exact one?
Any developer claiming that DLSS uses 3 GB of VRAM is either making stuff up/misinterpreting something that someone has told them (because they're not actually a programmer, more rather a lead designer for example) or they're referring to their own game's implementation of it -- and if it's the latter, then they're absolutely not doing it right.
I checked out Cyberpunk 2077 and The Last of Us, both on High quality settings, using PIX to record VRAM usage (not allocation).
CP2077
1080p = 5.8 GB
4K = 7.8 GB
4K + DLSS Perf = 6.0 GB
TLOU
1080p = 6.6 GB
4K = 8.7 GB
4K + DLSS Perf = 6.8 GB
In both cases, DLSS Performance mode sets the engine to use 1080p rendering, so comparing that footprint to native 1080p's gives one a good estimate as to how much VRAM the DLSS system is using. And as one can see, it's around 200 MB for both games.
Now, if we're talking about DLSS3, aka Frame Generation, then it's a slightly different story but not in the GB region. In CP2077, this sets the DLSS mode to Auto, which should use 1080p rendering when a 4K output is selected (according to the SDK documents). With these settings, the VRAM usage was 7.4 GB -- that's 800 MB over native 1080p.