Just got mine, I've owned an ES 100 before as well. Initial thoughts are that they sound like the ES100, I can't spot any major difference at least. But the ES 1000 do have an excellent EQ, the BTR3 lacks it atm. The ES100 also got a much better app/tweak settings and a 2,5 mm jack, the battery life is also a bit better. ES100 can power more demanding headphones as well.
But the BTR3 got some advantages over the ES100
+ Build, the glass/aluminium combo feels really nice, even tho i'd preferred an all glass build for better connectivity/feel.
+ USB C! Huge for me, then I dont have to think about an adapter/cable whenever I'm on the road, just need one USB C for both my phone and amp.
+ Buttons, far easier to find the right buttons for volume/pause/next... Even tho I'd rather change place on forward/back, so forward is on volume up and back is on volume down.
+ The clip feels really solid, much better than the clip on the ES100
+ Cheaper, for me its 30 euros cheaper
I haven't had the time to test the mic yet tho.
All in all, my dream device would be the software from the ES100 with the hardware from the BTR3, along with wireless charging and a bit bigger battery. But as off right now I'd go with the BTR3 if you want something simple to power your IEMS, and the ES100 if you drive headphones with a lot more settings tied into the app.
There's just something that's really weird with the BTR3, the app settings are tied into their music player app, why not release a separate app like the ES100?