Sentence:  The UK government has forced itself into a <WSD>trilemma</WSD> in which it has adopted three positions, only two of which can be achieved at any one time: to avoid a hard border within the island of Ireland; for the UK as whole to leave both the customs union and the single market; and to rule out any special arrangements for Northern Ireland in relation to a customs union and single market. 
Meaning of the word marked with <WSD> tags:  A circumstance in which a choice must be made between three options that seem equally undesirable or, put another way, in which a choice must be made among three desirable options, only two of which are possible at the same time.