“To be in use” vs “to be at use” vs ”to be of use”

Solution 1:

I believe these are confusing because of the involved preposition.

To be in use is similar to to be busy, occupied or utilized.

• I need to use the restroom, but it is in use right now.

To be of use means to be useful, of utility, or helpful.

• I finally got into the restroom and it is really of use to me.

I struggle to think of examples of to be at use like the one by GEdgar, but I've definitely seen and heard it used. The definition is restricted to to be utilized or used, and it does not mean occupied. I believe in use can be used in place of at use without confusion in these settings. The examples I can think of are rather unique:

• The electronic record system we are considering is at use currently in several other facilities without problems.