You only need three on one side still, but this is the way it works:
You get over the first two without a problem right, that's easy. But in order to get the third across to the other side you need atleast 4 people on start. 3 for the totems and plate, a fourth person that goes across. The way Bungie words the update it seems you can't cross a half-built/deconstructed bridge anymore. And because the other side still only has two you can't built the bridge from there. Furthermore, all people need to be on the other side for the ogres to spawn. As someone else mentioned, if the final person on the start side dies, it's over.