Hi,
The code which your wrote as below will not work,
lv_msg = wd_assist->if_wd_component_assistance~get_text( key = '003' ).
Do as below,
Create method in your assistance class with Import parameter of size CHAR3
and export parameter of size STRING.
Get the text into your string in that method and pass that to your message variable of lv_msg and call below standard code.
"report message
CALL METHOD lo_message_manager->raise_error_message
EXPORTING
message_text = lv_msg.
All the best,
Giri