[Torran]

Name all your vehicles with vehicle1, 2 ect,

pawn Код:

new Float:Cars[][1] =
{
  {vehicle1},
  {vehicle2},
  {vehicle3}
};

public OnPlayerSpawn(playerid)
{
  new rand = random(sizeof(Cars));
 
  PutPlayerInVehicle(playerid, Cars[rand][0]);
  return 1;
}

That should work,
Also use:

https://sampwiki.blast.hk/wiki/GetPlayerVehicleSeat

To see if the seats taken,
The code i gave is untested and i dont know if it will work