I agree with most forum members above. If the reader is really experienced and talented, they may pick up third parties from their sources however, third parties do not always indicate love rivals. If, for instance, as the above member said the third party works in a major career project with your POI, this will show up in the reader's cards and the reader may often misinterpret this as being a third love rival or interest which isn't always the case. Stalkers or should I say unwanted admirers of you or your POI may show up in readings as third parties but in reality, the admiration and feelings are only one-sided. I've had such stalker figures came out a few times when I read for myself and others and I found out after some time who or what the cards were referring to.
Unfortunately, I haven't found any method to distinguish exactly third-party interference e.g from family or fellow workers from actual third-party love rivals or unwanted admirers but there are some things giving you clues towards one direction or another. If, for example, you get three of swords next to a career/money card e.g three of pentacles or 8 of wands, it indicates third-party interference from work. If the card next to the first one is reversed, it probably means there is a ngeative interaction rather than a positive one.
I also had these card combos show up that indicate third party interference of a specific kind:
-Empress paired with three of swords. Indicates a heavy intervention of a motherly figure in your POI's life. Things become even more negative if the card empress appears reversed next to the three of swords.
-10 of swords next to three of cups (upright or reverse), I had this card combo personally turn out when someone's POI had suddenly left the querent for another person, leaving the querent devastated. Both of these cards are more emotional rather than practical and so they are most likely to indicate a third love rival.
-High priestess (reversed). This a quite tricky one as this card alone and reversed may indicate lack of proper intuition and spiritual connection between you as the female and your male POI (or your female POI if you are asking about a woman). However, paired with 3-themed cards or the 7 of swords, this is a strong indicator of a third-party mistress who appears quite mysterious and alluring to your male POI. The latter (7 of swords paired with the high priestess RX) often means that your POI (and especially if you are asking about a male) plans or is about to plan something with his mistress in secret e.g a secret trip together and the truth won't come out easily as both people are taking measures to hide it for their own reasons e.g they are both married or in long-term relationships.
Curious to know what other combos may indicate third party interference of a certain type so please share...