If you have class 3 firearms and someone other than you knows the combination to your gun safe you could both go to jail. This is a concept called constructive possession. If you have given someone the ability to access your class 3 firearms, you have violated the NFA. If someone has the ability to access your class 3 firearms, they have violated the NFA.
The best way to solve this problem is by creating a business entity or trust that allows the person to be in possession of the items. If you already own the class 3 items you will have to do another transfer from you personally to the NFA trust.
This is one of the many issues that should be identified and discussed prior to creating an NFA firearms trust.