This is the connection, such as it is, mentioned by Mr. Amdur. Three forms taken from two of the Omote no Tachi, and named Sho-Chiku-Bai by Ueshiba. The forms have completely different names in Shinkage Ryu, and different places in the pedagogy of Shinkage Ryu; they are not any kind of unit or trio there. So this is a case of Ueshiba taking the physical forms and (after modifying the forms to fit his principles) adding his own particular nomenclature. There are no forms called Sho-Chiku-Bai in Shinkage Ryu.
Are you certain that it was Ueshiba himself who called the above three forms Sho-chiku-bai
and not Hikitsuchi Sensei?
If the forms were general (not specially tailored for Hikitsuchi), one would expect to find them in Saito's kumitachi also. However, I think this is not the case.
There is pretty strong evidence that the terms aiki-ken
were never used by O Sensei himself, though this is what he supposedly taught in Iwama. There is a huge body of belief that it was Saito Sensei who faithfully transmitted the ken that O Sensei practised in Iwama, but it is the circumstances of the transmission that are in question.
Similarly, it can be argued that Hikitsuchi Sensei was also a faithful transmitter of what he had been taught by O Sensei, but that what he transmitted was not the sum total of what O Sensei himself called Sho-chiku-dai