Captain & Tennille 1979 Do That To Me One More Time

Out of all the different readings that I do, more often than not I see the Three of Cups reversed come up in relationship readings when there is a third person involved. 

Often it is a reflection that you, the querent, are that third person and may be involved in an affair or dealing with an unfaithful partner. 

When this card appears in a reading, look to the other cards for more detail on the nature of the relationship. 

If you see the Seven of Swords, there is a likely betrayal of trust and possible cheating; 

if you see the Five of Swords, it is likely that you already know about the other relationship but you are trying to ignore that it exists; if you see the Devil, think carefully about the nature of the relationships as they are likely to be hedonistic and generally unhealthy to those involved. 

It is unlikely that this situation is sustainable as ‘three’s a crowd’, thus something has got to give.

https://www.biddytarot.com/tarot-card-meanings/minor-arcana/suit-of-cups/three-of-cups/