TechPowerUp published a review on the Etymotic ER3XR
A quote from the article:
Etymotic's ER3-series in-ears aim to bring their ER4 design into a lower price category. Priced at $179, the ER3s are nearly half the price of the ER4-series in-ears, and their specification sheets show that they are not far off. We take a good look at the new in-ears to see how they fare against their bigger brethren.Etymotic ER3XR Review
Etymotic was one of the first companies to produce a really great-sounding in-ear with the ER4S. That was over 20 years ago and now, they have released a new "baby" ER4 dubbed the ER3. Last year, we had the pleasure of reviewing the new ER4XR and ER4SR which both lived up to their name and then some. The ER3 is Etymotic's attempt at creating an ER4-type in-ear at a lower price. Where the new ER4 models retail for around $349, the new ER3 models will retail for just $179.
Since Etymotic has never been much for lowering their quality standards in terms of sound, they had to look elsewhere. The ER3 ships with a smaller bundle which depending on your ears and needs might go by totally unnoticed. In order to get ER4 tech down in price, some of the less essential things have clearly either been omitted or had their acceptance criteria reduced.