As per my knowledge,

Stock Input Account is an interim account for parking the "accounting payable"during 'Receive Products" because later this will get knocked off against the Vendor (Accounts Payable) during creation of Vendor Bill. 

So this account type should be 'Current Liabilities'

Similarly Stock Output Account is an interim account for parking the "accounts receivable" during "Shipments" because later this will get setoff against the Customer (Accounts Receivable) while creating Customer Bill.

So the type should be "Current Assets"

Stock Valuation Account is nothing but the account showing real value (current value) of assets. So this must be Current Assets.

So the accounts must be linked as follows:

Stock Input Account-> Current liabilities

Stock Output Account-> Current assets

Stock Valuation Account -> Current assets