I'm not sure where you're getting the 3% number from. If you read the New York Times maps page data[1] and just divide the total deaths by total cases you get 3.7%, but that isn't the death rate because it doesn't take in to account a lot of factors.
Instead, there's a good article[2] about how bad the virus will get, that says estimates put it in the range of 0.1% to 3%. (With the seasonal flu being somewhere around 0.1%, but the Spanish Flu of 1918 being somewhere around 3%.)
That said, the general intuition that less reporting of cases means that some part of the death rate is over-inflated makes sense. But also consider that there are factors that go in the other direction. Anyone who currently has the virus but has not recovered or died can be under-inflating that rate. And there are likely lots of other factors, which is why articles often mention that the "true" death rate cannot yet be calculated.
(Also the death rate varies a lot with age, and isn't the only factor in determining the "badness" of a virus. Which that second article goes into more depth on—things like how contagious it is, and how early a vaccine can be developed. So the average rate being half of the current numbers doesn't necessarily mean the concern is over-inflated as well.)
[1]: https://www.nytimes.com/interactive/2020/world/coronavirus-m...
[2]: https://www.nytimes.com/interactive/2020/world/asia/china-co...