Don’t, then - nobody told you to do that.
Modbus “sucks”, if you like. It’s a decades old idiot protocol, never intended for non-professionals to even think about. It’s very flexible, it’s widely used for all manner of things, it’s very fiddly, it’s hard.
But you’ve chosen to use Modbus, because no-one has written an SMA specific binding for openHAB (there’s not much need), and you’re not going to write your own.
That’s okay, SMA provide some assistance by following SunSpec standard.
Follow the docs.
User note -
Note the ID for your TCP bridge Thing.
I think at the moment you are trying to fit unSpec shaped openHAB Things onto SMA’s generic modbus at ID 3, rather than SunSpec shaped Modbus at 126.