All of us know that "from now on" means starting from now, for example:
From now on, all of us have to turn our mobiles off.
But is this term assignable to such phrase?
From lesson three on... (I mean starting from the lesson three...)
"From now on" means "from this moment". More spesifically, it means strating from this moment or strating now. And it automatically implies that it will be that way into the future and not just happen in one little moment and stop and never reoccur.
"From now on" doesn't mean "starting from now." (You would just say, "Turn your phones off, now") It's more accurately used as "In the (near or distant) future, when one finds oneself in the same (or similar) situation."
"From now on" means forever, or at least for the foreseeable future. "I'm going to keep my front door locked at night from now on." You wouldn't keep your mobile phones turned off forever! I would expect you to say "Let's turn off our mobiles now" (just for the duration of our meeting, etc), or "From now on, let's turn off our mobiles during meetings."