split_fusion_qgis

The QGIS expression is designed to split a string (found in the "Code_ouvra" field) by underscores ('_'), and then recombine the first three elements of the resulting array into a new string, again separated by underscores.

```qgis
array_to_string(array_slice(string_to_array("Code_ouvra", '_'), 0, 2), '_')
```

This expression does the following:
1. `string_to_array("Code_ouvra", '_')`: Splits the "Code_ouvra" string into an array using '_' as the separator.
2. `array_slice(..., 0, 2)`: Takes a slice of the array from the first element (index 0) to the third element (index 2).
3. `array_to_string(..., '_')`: Converts the sliced array back into a string, with elements separated by '_'.