If you are trying to send floating point data in a form that the Simulink will be happy with -- one of the binary formats that RadioEngineer listed -- then trying to encode the integers 20, 30, 40 as a string "20 30 40" is not the right way to go about it. Read the other thread I referenced, and ask specific questions.

In that other thread, I already said which pack formats are needed in order to properly encode floating point numbers; you concatenate those together (with whatever separators that the Simulink block is expecting) and send them as a single message.