One 3-way crossover that sticks out in my mind is when NBC did a special night of The Golden Girls, Empty Nest, and Nurses (I believe in 1992 during GG's final season), where there was a full moon and there was an ongoing theme with the fact that there was a full moon out. I remember Rue McClanahan crossing over to Nurses, but can't remember if she or Betty White crossed over to Empty Nest as well. Either way, I liked that trifecta of shows with frequent overlapping crossovers.

Just did a quick Wikipedia search. Charlie, the neighbor from Empty Nest also crossed over to Nurses that night. Looks like Nurses had frequent GG and EN guest-stars.

I don't remember the full moon..but I remember there was a time a hurricane came and it impacted all 3 shows...seems like there was a 4th too.

I loved the Law and Order/Homicide crossovers. And the appearances of Mothership characters are really the strongest thing SVU ever had going for it, especially after McCoy became DA and kept showing up to bitch the ADAs out. Even Fred Thompson was good for those scenes.

I second that emotion!!!