Both Rashi and Tosfos are trying to answer a problem with the Gemara which seems to imply that the keys are never entrusted to an emissary. However, in other places (see, for example, Ta'anis 23a) the Gemara specifically mentions the key for rain being in the hands of Eliyahu. Rashi therefore suggests that the Gemara means that the messenger cannot have all three keys at the same time, although the Gemara implies that they cannot even have two keys at once, since it says that it would not be right for the disciple to have two keys when the master has one. Tosfos therefore rejects Rashi's approach and suggests that the key cannot remain by the emissary forever (I suppose it means for the duration of his life, but Eliyahu is still technically alive).

Rashi in Sanhedrin 113a says that even one key is not given to an emissary, but Eliyahu was "appointed to oversee" one of the keys. This is a different approach from Rashi in Ta'anis (which is reputedly not written by Rashi but by one of his disciples). (See Maharatz Chayos.)

Yoel Domb