I am a complete beginner with ARCore and I would like to ask if it is possible to display 3D objects using 3D coordinates and having the user as the origin? For example, having (x,y,z) values of (0,3,0) will display the 3D object to the right of the user.
First of all: There is no native implementation in ARCore for that!.
But you're not completely lost. There are some very useful libraries in Android like appoly. I've used this fork from Eric Neidhardt.
For iOS I only know this library based on ARKit.
You can also make a Unity3D app in C# and use AR Foundation and Vuforia Plug-in. This way is platform-independent, but imho testing is a pain.
You see, there are a few options out there, but be careful that none is really accurate!