The KZ ES3 is essentially identical to KZ ZST, with a slight bump in the upper-treble that makes the piercing issue a bit more prominent. The slight difference doesn't warrant a dedicated review, so the review is identical to ZST.
I personally find ES3's shape a bit more comfortable, but that's a personal preference.
Hard paperboard + soft paperboard
S+M+L, starline, black, ø4.5mm bores
Plastic (glossy), ø5mm nozzles (ø4mm bores)
2-pin (custom 0.75mm)
Plastic (matte), single-button
TPE, soft strain reliefs
Silicone (grippy), 127cm, soft, mildly bent, memory-wire ear loops
TPE, 90° angled, short, soft strain relief
I always find the memory wires to be extremely annoying. It takes too much time and effort to position the earphones properly into the ear and even when they're inside, they don't sit flat against the ear and stick out a bit and are very easily pulled out.
The bass is very balanced and controlled, but it's also underwhelming and overrun by the super-bright highs. It's more than enough for contemporary vocal-oriented music, but for electronic music this is just not enough. Depth-wise, they are similar to ZS5 but slightly shallower.
The mids are highly impressive, and actually very similar to ZS5, to the point where it's really hard to tell them apart. The detail is quite extraordinary, but I feel like the ZS5 manages to pinpoint microdetail quite better than ES3. Along with the great vocal clarity, comes some unsatisfying pierce that afflicts vocals, especially female vocals, and high-pitched instruments.
They highs are very sharp and crisp. They are also extremely piercing. Depending on how sensitive your hearing is, you might feel discomfort and fatigue after using these earphones for a while, especially if you're listening on medium/high volumes. Superb calrity and tonally accurate, except that the treble is very bright, far brighter than what I would consider enjoyable.
