It is cheating, and she KNOWS it is cheating because she deliberately did it at a time when she was sure you wouldn’t be coming home (and if you think this is the only time it has ever happened you are too naive to exist in modern society) which means she knows damn well that what she was doing is wrong and violates your relationship boundaries. Personally I would break up with her and leave. If you want to give it one try, then either you and Laura find somewhere else to live immediately or Kate does – your relationship will not survive all three of you living together because you will never trust that they aren’t fucking around every time you leave the house.
If you’ve both consented on a monogamous relationship, it definitely is cheating.
Definitely cheating, up to you what it means. Talk to the GF and sort it out.