Thanks, and now I see the problem and how to solve it. That's why when I press the step 3 button, sometimes I got 4 pulses. I tried to sync the start of pulses of the astable 555 using its reset pin to start the pulsing from the 555 mono-stable trigger pins through AND gates but it didn't quite work as I planned.asbf - thanks for the effort, but you're not quite there yet. The monostables have to hold the astable 555 in reset to guarantee that it always starts with a complete cycle. In other words, the gating has to come before the astable, not after it.
So, I'd leave it for the OP to solve this synchronization problems.