Concurrent task processing in LPC2148

Hi all,

I am using RTX in LPC2148, I am using 3 tasks with different priority. I want to run the tasks periodically like task_1 run for every 10ms,task_2 runs for every 20ms and the third one for every 50ms.
Is this possible to run a task concurrently means i want to how to configure it. I try the below method

__task void task_1(void)
{ os_itv_set(10); while(1) { os_itv_wait();

/* my code goes here*/ }

}

same procedure i followed for remaining tasks, but i am not get what i am expected. Is there is any other ways are available in RTX.

Thanks in advance