Changing to the left lane is a separate maneuver, and the signal is switched off immediately after completing it. When you get ahead of the truck you are driving straight, and a signal left on would confuse others.
Under the Rules, a turn signal must stop immediately after the specific maneuver is completed (clause 36). Overtaking consists of two separate maneuvers: changing to the left lane, and then returning to the right. So the left turn signal must be switched off immediately after you enter the left lane. This keeps you from misleading other drivers while you drive straight and get ahead of the other vehicle.
Option 2. Leaving the left signal on while you drive straight ahead breaks the rule and gives the impression that you intend to turn even further left.
Option 3. The moment to switch signals off is strictly set by the Rules so as not to mislead others; it is not left to the driver's discretion.