I don't have er3se but I had er4se er4xr and have er2se, hf5 with and without ptos adapter.
I understand that you are comfortable with flat bass. So basically er2se still has flat bass of all the "s" models. Highs on er2se is more than hf5 stock, and less than er4sr. It's similar to er3se. But er3se has rolled off highs on stock. You need resistor in series to bring up the highs. Otherwise it's similar to hf5 stock. Other than that er2 is dynamic driver and according to measurements it has much lower distortion especially at normal listening volume. With low output impedance amplifier it sounds clean transparent and smooth, er4 will sound in comparison a little dirty in the highs.
Summing it up.
Stock: er2se has more highs than er3se.
If er3se has resistor adapter like ptos, er3se will have more highs and less lows.
Choose for your taste.