Billy and Mandy are only similar because the one kids handicapped and the other's a girl.
Other than that you could say any show is like Billy and Mandy. Edd, Ed, and Eddy was that because Edd was handicapped, Songebob is like it because Patrick is handicapped. I could see if the third character was say the Grim Reaper, but as far as I know, it's not much like the show.
From what I picked up of the first comic, it goes like this:
Jenny/Grim - Often off doing something else, does not enjoy Jimmy's/Billy's presence. (Not sure about Jenny, yet, since they did not meet in the comic)
Willy/Mandy - Spends a lot of time at Jimmy's/Billy's house, does not enjoy his presence but tolerates it.
Jimmy/Billy - Your regular blockhead, very oblivious.
Then we could compare it to Spongebob, as you mentioned:
Squidward/Grim/Jenny - Grumpy, tries to avoid Spongebob and Patrick
Patrick/Jimmy/Billy - "Your regular blockhead, very oblivious."
Spongebob, Simply does not fit the equation.
Or we could compare his comic to Ed, Edd, and Eddy, as you also mentioned:
Ed/Jimmy/Billy/Patrick - See above
Edd, does not fit the equation, very tolerant of both characters, smart, does not encourage Eddy's schemes, but helps anyway.
Eddy, also does not fit the equation.
Each one has at least one unique character except for, of course, Animated.
On that note, it was either
episode 45 "Billy Idiot", or
episode 52 "Hill Billy" of The Grim Adventures of Billy and Mandy that had a very similar plot and and started in an almost identical way to the comic you currently have posted. I remember comparing it to one of those two in the original thread as well when you told us about the plot.
I liked your comic, and I like your art, but it's not very original.
