You should NEVER be in neutral at speed and coasting. This is bad driving habit. You should be downshifting or upshifting as your speed changes. If you continue in this fashion, you are increasing your chances of doing harm to your car.
Learn to gear change with road speed and learn to rev match.

As for your scenario, you will continue to eat your syncros away. If you are coasting in neutral from 40 down to 30, you will be idling your engine. If you slam your tranny into 3rd at 30, your engine speed and road speed is not even close to what they need to be. If you had been downshifting while you were slowing down, then when the light turns gree, you would be ready to just give it gas and continue.
Like I said, you need to learn about driving stick either from a driving school or from someone who knows how to teach it to you.