One small correction. At the library definition, you don't add the ".dll". You never include a .dll but the .lib file.
In any case the correct is the following:
LIBS += -letel3rdparty
The library file (.lib) is used from the linker and the .dll is needed at runtime. So using the above line is only the half job. You have to define as well where to put the .dll or else it's going to compile fine but it's not going to work when you try to run it. For that reason you need also the following lines:
Now you should be able to use the library.
addFiles.sources = etel3rdparty.dll
addFiles.path = /sys/bin
DEPLOYMENT += addFiles