The answer is: well, it depends on the size of the intersection and the size of the vehicles.
Basic Turning Left Concepts
If you enter an intersection when a light is green, then you’re allowed to leave the intersection when it is safe (the light may be green, yellow, or red). I assume we can all agree on that much.
In terms of how many vehicles can enter an intersection at a time, consider the definition of a yellow traffic light. Any vehicle that is not yet inside the intersection and is facing a yellow traffic light is required to stop before the white stopping line as long as it’s safe to do so.
Therefore, a car that has not entered the intersection on a green light is legally not allowed to enter the intersection and turn left once the light has gone yellow or red. Car 2 from its position can not legally turn left once the light has gone yellow:
Additionally, stopping on top of the marked crosswalk is a strange place to be. In this position, you are often not yet inside the intersection, so you can not leave the intersection legally once the light has gone yellow or red. And, it’s illegal to reverse once you’re in this position; and potentially irate pedestrians would ensue. Car 3 should not stop on the crosswalk, in case the light turns yellow:
On a larger intersection, there is no reason why more than one vehicle can’t pull into the intersection (although, YOU DON’T HAVE TO pull into it if you don’t want to). Car 2 is legally inside the intersection. Car 3 is waiting behind the white stopping line in case the light goes yellow.
There is nothing that says you must pull into the intersection if you are in the second (or third) vehicle. Use your judgement, common sense, and consideration for the individual location.
Here is a not perfect, but not too bad example (closest thing I could find in my 1 minute search for a pic). Clark Drive and East Broadway in Vancouver is a larger intersection. Car 1 has pulled forward an adequate amount in order to line up with where he’s going to turn and to ensure a short turn into his new lane (better if he keeps his car perfectly straight in case of a rear-end collision though). Car 2 (the van) is inside the intersection, and car 3 is waiting behind the line. Overall it looks fairly decent:
When the light goes yellow, Car 1 and 2 can leave the intersection, but Car 3 can no longer move.
This, on the other hand, does not look decent at all:
What is car 2 doing in that position? It is not far enough forward to be considered ‘in the intersection’ and if it turned with the red car once the light went yellow it would be an illegal turn. If it stays there when the light goes yellow, it’s blocking the pedestrians. And, it is illegal to reverse on a crosswalk.
BC Motor Vehicle Act:
165 (1) If the driver of a vehicle intends to turn it to the right at an intersection, the driver must cause it to approach the intersection and then make the turn as close as practicable to the right hand curb or edge of the roadway.
(a) cause the vehicle to approach the intersection in the portion of the right side of the roadway that is nearest the marked centre line, or if there is no marked centre line, then as far as practicable in the portion of the right half of the roadway that is nearest the centre line,
(c) after entering the intersection, turn the vehicle to the left so that it leaves the intersection to the right of the marked centre line of the roadway being entered, or if there is no marked centre line then to the right of the centre line of the roadway being entered, and,
(i) may cause the vehicle to proceed straight through the intersection, or to turn left or right, subject to a sign or signal prohibiting a left or right turn, or both, or designating the turning movement permitted,
(b) a pedestrian facing the green light may proceed across the roadway in a marked or unmarked crosswalk, subject to special pedestrian traffic control signals directing him or her otherwise, and has the right of way for that purpose over all vehicles.
(a) the driver of a vehicle approaching the intersection and facing the yellow light must cause it to stop before entering the marked crosswalk on the near side of the intersection, or if there is no marked crosswalk, before entering the intersection, unless the stop cannot be made in safety,
(a) the driver of a vehicle approaching the signal must cause it to stop before entering the nearest marked crosswalk in the vicinity of the signal, or if there is no marked crosswalk, before reaching the signal, unless the stop cannot be made in safety, and