Agreed, you need to be specific in what you are looking for.
If your question is "how do I measure impedance in a three winding transformer", there is a very specific procedure and equations in ANSI/IEEE C57. Most of us are familiar with a two winding test procedure, but the 3 winding is more complex. For those of us who perform short circuit studies on 100MVA three winding xfmrs, this is necessary to understand. I can provide you with the specifics if you are serious, but we are only talking about a little 225VA unit.
Don't count on a highly inductive result either; that's reserved for the big boys. A little xfmr like this will actually be mostly resistive.
If you are only interested in the inductance number, you will still have to go through the impedance method. Then again, I may not understand exactly what you are looking for.