I'm not sure about the "three students" question, but below is a list of past and present Menkyo Kaidens of SMR, both Tokyo and Fukuoka in no particular order.
Otofuji Ichizo (deseased)
Kuroda Ichitaro (d.)
Nakajima Asakichi (d.)
Kaminoda Tsunemori
Pascal Krieger
Matsui Kenji
Nishioka Tsuneo , Who I believe is the most senior living (and practicing) student of Shimizu Sensei.
Matsumura Shigehiro
Donn F. Draeger (d.)
Quentin Chambers
Phil Relnick
Nobuko Relnick
Yoneno Kotaro
Hiroi Tsunenji
Nakajima Asakichi (d.)
Namitome Shigenori
There are plenty more but these ones are just from memory. Hope that helps (a little)
Fredrik Hall
"To study and not think is a waste. To think and not study is dangerous." /Confucius