This was one of the most disappointing things for me. Funny you should mention this because I spent too much time last night comparing the frunks of S vs 3. The S is much deeper and I know it got smaller with the HEPA filter, but there are pictures of people curled up inside it. That's nothing to sneeze at and is perfect for stuff like groceries.
The 3's frunk is extremely shallow. It's hard to get an idea of scaling, but it's wedge-shaped and it looks like the shallowest depth is maybe 4-5 inches? Possibly 6? That's practically nothing. Sure, it tapers up to maybe 10 inches, but at that point it hardly qualifies as storage space. I might be able to fit a couple bags of groceries in there, but that's it. Very disappointing.
I really wish they hadn't reduced the height of the hood so much. They talk about increasing visibility, but of all the visibility issues I can think of when I drive, being able to see an extra foot of ground in front of me is the least of my worries. Being able to see out over your car and onto the ground is just not a problem that needs fixing. By doing this, Tesla compromised frunk cargo space and also aesthetics because it makes the hood look stubby and anemic. I'm quite positive this increases aerodynamics though, so I'm sure that's the driving reason behind the poor hood design.