How to fix objects?

I need to insert some virtual objects in an internal environment, but I need the position of these objects to be fixed. I have tried to use markers with the vuforia, in an environment they get small and are difficult to recognize. I'm thinking of using ArCore, because in some videos it seems to do exactly what I'm looking for, but I do not find any tutorials and I'm in great need of help. Can anyone give me any hints, give me some tutorial on this? It is a short path in a corridor, I need to position 3 objects along this path, but the position has to be fixed so that both the way and the back, the person sees the objects in the same place.
It's for a challenge that I have to solve in college.
Thank you all!

 

Can someone help me?