I'd like to have a go at responding to your question. First, we do not know and that information is not published, to the best of my knowledge.
My view is that the Queen has offered certain tiaras to Kate who has made her choices, not necessarily all at one single time.
My guess is that all three tiaras we have seen Kate wear so far are housed in a jewel room at BP. Kate probably has to "requisition" for one when she needs it. Staff would organise its delivery and sign for it at each handover point in the delivery chain. I expect the Lover's Knot was sent from BP to KP where Kate dressed for the occasion. Quite possibly the tiara spent a night or two at KP in a safe in her dressing room.