Untill the HK 121 will fully replace the MG3 it´ll take many years. AFAIK there are only a few prototypes being tested right now and not even our infantry units who usually get high-tech stuff such as the MG4 and other IdZ stuff first, have laid even their eyes on that new machine gun. It´s the Bundeswehr we´re talking about...
Little annecdote from a former comrade of mine who was in a medic company before he joined the MP:
The G36 was supposed to replace the G3 as standard assault rifle some times in the late 90s IIRC. When said comrade had his basic training 5 years ago he was still trained with the G3, while most other units already had their G36.
So when it comes to new, fancy equipment it takes a long time untill all Bw units are equipped. And the lower a unit is on the supply chain the longer it takes to get new stuff.
So it might take another few years untill you could arm your fancy Opel with a surplus ex-Bundeswehr MG3.