Because it happens. But the pre-requisite is that the school must trust the geo3 officer enough to stretch their deployment a bit.
First and foremost, the school should not be left worrying about their T&L. Not 100% perfect, but already of a high standard and able to independently reflect and constantly seek self-improvement.
Then they must show ability to handle more things on their plate. Some schools test their promising geo3s by deploying them similarly to a geo4 or 5. This means double committees (including potentially 2ic of one), higher profile committees like student leadership, discipline, exams, etc., key roles in functional comms, CCA head, special programmes coordinators, sending them for cluster/zone/national sharing or to join NLCs.
And we all know that to be promoted, you must show potential to work effectively at the higher level.
In order to get the school to 'overload' a geo3, the school must trust that the young officer has his/her basics down. The basics must come automatically and with less effort compared to a BT. If they are still shaky during lesson
obs, resources messy, cannot keep up with T&L responsibilities, tardy with single comm/single CCA work, then they can forget about being over-deployed with the intention of testing their readiness for promotion.