Splitting a mnemonic code or seed like that is not a good idea at all.
Using a secret sharing scheme is superior since it does not leak any information about the secret at all.
Right, but the secret sharing scheme has two main issues: 1) amnesia (you forgot where did you store one of the 3-4-5 pieces) and 2) what if one of the pieces is somehow damaged.
This problem can be solved by making duplicate or several copies of each piece. Since a single piece on its own cannot be used to recover the seed there is no risk of funds loss if you suspect one of the copies was stolen, since you know where the rest of the pieces are and can just assemble them together and broadcast the bitcoins to a new address. Then you create new secretly-shared papers because the old ones automatically become invalidated after the seed is sweeped, by virtue of having no balance in them.