Hey Soundstige,
I'm glad you're excited about the ER3 series. After working on them for a while, I'm pretty excited about getting them out as well.
The new ER3s are intended to build off of the new ER4 series at a more affordable price point. Basically, the same family, but with a different response, less strict channel matching and made in Asia. They still use balanced armatures, precision machined aluminum bodies and detachable cables with MMCX connectors.
Basically, they are meant to be part of a new family of earphones. While the cables on the ER3 and ER4 series are different, they are both functionally interchangeable. This will be a bigger deal in the future when we introduce other cable options. For example, we are going to bring the balanced cable out as an ER4 accessory, but it will work on the ER3. Likewise, we are working on a microphone cable for the ER3 but it will work fine on an ER4 if one was interested in that.
As far as sonics (which is of course, the most important), the ER3SE is a bit more accurate than the HF5. In some ways, it's a middle ground between that and the ER4P, but the 4P had some other advantages (again, matching, etc). The difference this time around is that we've introduced an ER3XR along with it for those that want a bit of a low end kick.
I don't think it's going to make most folks sell their ER4SR/XRs but I think it's cool in it's own right and hope folks dig it. I'm assuming we'll all know soon enough.