There is not a problem. to gain access to the 3rd dungeon, you will need to find the 3rd battlefield key. every time you find a key, another dungeon will unlock for you.
In act 1 there are 3 battlefield dungeons. and 3 keys
In act 2 - 4 there are 6 dungeons and 6 keys.
THe battlefields generate random creatures the first time you enter it in each act, so sometimes there will be more than one big monster in the 3rd level of a dungeon and sometimes some will go empty.