Yes, you just have to use the Reflecting Mirror on it. To get the Mirror, just:
>Bring Landorus in its Therian Forme to Abundant Shrine. -Bulbapedia
Unfortunately, you need the Reflecting Mirror on to avtive Landorus-T, so unless you use AR or any hacking device, it won't work. Or trade on GTS.
So yes, you do need a 3DS in the end. You need the Dream Rader app (credit to Unreconizable) to get Thunderus in it's Therian form.