How Do I Send 3D Reference Files To Back?

Mstn v8i SS4

We design water systems. The dgn is 3D. We attach the sewer design to our dgn's as references. The sewer dgn's (done by others, so we cannot modify) are also 3D. The sewer always seems to plot on top of our work. How can I send a read-only 3D reference file to the back?