The "Clk:xxxs" pulses have predefined intervals that cannot be changed.
To create a clock pulse with arbitrary period, use a self-reseting timer as follow:
T1 Fn_#1
|---||---------{dCusF}
| T1 T1
|---|/|---------(TIM)
Timer T1 is a self-reseting timer and you can use its contact at any circuit before it as a clock pulse to trigger the custom function. Make sure that the place the self-resetting circuit last. In your case, T1 should have a preset value of 30 to get 3 seconds interval.