For this case, I have three things in Items which are all different (and there is a check to ensure that there are exactly three)
If I check Distance for FIRST_IN, LAST_IN and THERE_IS_IN with three different values, THERE_IS_IN has to be satisfied by the middle Item since the other two are accounted for.
I am taking your point that I should endeavour to eliminate any ambiguity in my tests, and I will pay more attention to this in future (including the next version of this test) but I think that THERE_IS_IN can be used here without any problems.

I am changing to FIRST_IN and LAST_IN for the relevant elements because of your comment.