The 3000 is the mark of the X axis on an OFP map and the 1500 is the mark on the Y axis of a map. Z is for the height.
For example:
unitname setpos [ 3000 , 1500 , 50 ]
..."beams" a existing unit/object to the position 3000/1500 in a height of 50 meters.
You can get the coordinates by opening the mission.sqm and search for the players coordinates for example.
You can also use:
unitname1 getpos setpos unitname2
...this "beams" a unit/object to the position of an other unti/object. Mostly this is used together with the "invisible H" (setpos unit to position of invisibel H).
Setpos ist also used to simply change the height of an unit/object.
Initline:
this setpos [(getpos this select 0, getpos this select 1),1]
Now the unit/object is created in a height of 1 meter.
Greetz
Plage