My answer would be the following:
Y has a non controlling share but does have significant influence.
For seperate financial statements apply IAS 27.38: Measure at cost or in line ith IAS 39 (available for sale: apply IFRS 5)
For consolidated statements apply IAS 28: Equity method. The investment is intially recognised at cost and the carrying amount is increase or decreased based on proportion in profits/losses, impairments and distributions.
IFRS 3 would only apply in case of controlling interest when the subsidiary would be consolidated.
I hope this helps.