The rules say that if you end your movement in an illegal space (e.g. one that is occupied), you must "put your miniature in the last legal position you occupied, or the closest legal position, if there's a legal position that's closer."
Now, if there was simply no spot that is 3x3, then it may have to enter the rules of squeezing into a 2x3 or 2x2 spot (you can only squeeze into spaces at least half your normal size, rounded up). When squeezing, a target is at -4 to attacks and AC.
Of course, I'd question whether or not it is realistic for the small creatures to remain still and take the charge vs getting the hell out of the way of a (to them) ginormous elephant. If it were a bunch of goblins or kobolds, they'd scatter because it isn't in their personalities to just sit around as roadblocks.
Of course, if it was a room full on animated dog-sized chess pieces (mental note: use this in a game), then no, they'd probably not move. But either way, not because of tactical rule metagaming, but because they are not afraid.
The GM could also allow a little forced shoving around in the room to make the elephant have at least a 2x2 square to squeeze into.
If memory serves a creature 2 sizes smaller than another can occupy the same square.
Square Occupied by Creature Three Sizes Larger or Smaller: Any creature can move through a square occupied by a creature three size categories larger than itself.
A big creature can move through a square occupied by a creature three size categories smaller than it is. Creatures moving through squares occupied by other creatures provoke attacks of opportunity from those creatures.
if the room was large enough for a squeezed elephant (2x2 or larger) it should have been able to trample the room then leave if necessary.
Trample (Ex) As a full-round action, a creature with the trample ability can attempt to overrun any creature that is at least one size category smaller than itself. This works just like the overrun combat maneuver, but the trampling creature does not need to make a check, it merely has to move over opponents in its path. Targets of a trample take an amount of damage equal to the trampling creature's slam damage + 1-1/2 times its Str modifier. Targets of a trample can make an attack of opportunity, but at a –4 penalty. If targets forgo an attack of opportunity, they can attempt to avoid the trampling creature and receive a Reflex save to take half damage. The save DC against a creature's trample attack is 10 + 1/2 creature's HD + creature's Str modifier (the exact DC is given in the creature's descriptive text). A trampling creature can only deal trampling damage to each target once per round, no matter how many times its movement takes it over a target creature.
the words 'moving through a square' and 'trample' are the links to the rules i quoted in my post. scroll up and click on them and that will bring you to the paizo prd
The elephant has to begin its turn in some legal space (its starting square), and begins its trample move through oponents. If it ends its move on an illegal space (ie, an opponent's space) it moves back to its last legal position.
So it could, in the OP's situation, get moved backed all the way to its starting square.
Bald Man and Morgen:
A creature cannot occupy the same square as another creature unless the other creature is tiny or smaller. The 3 sizes larger rule covers moving through, not staying in, another creatures space.
Koaer, the relevant rules are on CRB p193.
- Gauss