20ns pulse generation at rising edge

So you need your trigger to be aligned with the rising edge of the 3 MHz clock and then go low 20 ns later? Did I understand you correctly? This is not possible with the clocks that you have.

However, you can make a 20 ns pulse when a rising edge on 3 MHz is detected using the 50 MHz clock. The issue with what you are trying to do is that the 3 MHz and the 50 MHz clocks are not synchronous.

/Boris