The kankers are meant to represent three different types of unappealing girlfriend archetypes >Marie
Manipulative: Marie's only interest in Edd is his naivete she sees him as a sucker that she can tease and lead on to get whatever she wants from him but would probably just cuck him and leave him as soon as he wises up or has nothing more to provide.>Lee
Ballbusting: Lee is a straight up spiritcrusher she sees Eddy as a challenge because he is almost as stubborn and forceful as her so she wants to break his will until he fits what she wants him to be, once she has successfully changed him he will cease to be interesting and she will dump him.>May
Smothering: May contrary to the other two actually seems to genuinely like Ed (And Edd that one time) because of shared interests but in her case she is absolutely obsessed to the point of being obnoxious she wants Ed to spend all his time with her in neglect to anything else.
All three were aparently raised by their single mom which from their descriptions seems pretty bitter about men in general and may have filled their heads with unhealthy ideas of what relationships should be like.
That said the reasons the eds don't like them is just because they are weird trailer trash that always mess with them.