1) It does indeed seem that a number of Rishonim disagree with the Rosh. A major question on the Rosh is that the Mishnah above (45a) states, "If he shaved on one of the three Korbanos he was Yotzei." The Tosfos Yom Tov there writes that this is true according to everyone. He seems to mean that this is true according to all the opinions mentioned in the Mishnah on 45a. Now, one of the opinions cited there is Rebbi Elazar. However, one could say that this is "Rebbi Elazar" without a "Yud," while Rebbi Eliezer on 47a is with a Yud. However, the Rashash on Tosfos below (63a, DH Man Tana) implies that there is a text on 45a that reads "Rebbi Eliezer" with a Yud, but he adds that even if the text reads "Elazar" we still have not found that Rebbi Eliezer disagrees with the Mishnah on 45a.

2) I will leave the above problem without an answer for the moment, and now point out that the Rosh is not alone on this point, and in fact he probably received his opinion from Tosfos 47a, DH v'Nitma, who writes that Rebbi Eliezer maintains that since it states in the Gemara (46a) that according to Rebbi Eliezer it is only after all the Ma'asim that the Nazir may drink wine, this means that he also may only shave after all the Korbanos have been brought.

The Chazon Ish (Even ha'Ezer 141:4, end of DH 45a) writes that the opinion of Tosfos is that since he is not allowed to drink wine until he has brought all three Korbanos, he is also not allowed to shave.

3) Before returning directly to the opinion of the Rosh, I think that it is worthwhile to look at how the opinions of the different Tana'im in the Mishnah on this issue relate to each other. The Tosfos Rid, on the Mishnah 47a, writes that Rebbi Eliezer in our Mishnah is the same as the Tana Kama in the Mishnah, end of 45b, who states, "And afterwards the Nazir is allowed to drink wine and become Tamei to the dead." The Tosfos Rid writes that this indicates that it is after the Korbanos and the shaving and the waving of the Korbanos, which is the opinion of Rebbi Eliezer. Then Rebbi Shimon, at the top of 46a, states that if one of the Korbanos has been brought, the Nazir is allowed to drink wine and touch the dead. The Tosfos Rid writes that Rebbi Shimon is identical with the Chachamim, on 47a, who argue with Rebbi Eliezer.

4) So we have a source from the Mishnah that to allow the Nazir to drink wine and enter the cemetery, one must bring all the Korbanos according to Rebbi Eliezer. The Chidush of the Rosh, following in the footsteps of Tosfos (47a, DH v'Nitma), is that one can extend this idea to shaving -- that he is allowed to shave only after all the Korabnos have been brought.

5) I am not sure what is so Mechudash about saying that all the Korbanos are like one, and I do not see that there is a big difference between saying it that way, or saying that all the Korbanos are needed on the day of Gilu'ach. Just for the sake of illustration, this could be compared to someone who wears only three strings of Tzitzis. The Mishnah (Menachos 28a) states that the four strings are one Mitzvah. If someone wears less than four strings, he has done no Mitzvah. The same thing goes for the Korbanos of the Nazir. The three Korbanos are like one, and if one Korban was missing he has achieved nothing.

6) Let's look more deeply at the explanation of the Chazon Ish (Even ha'Ezer 141:4, that I mentioned briefly earlier).

a) The Chazon Ish writes that the Mishnah on 45a, "if he shaved on one of the three Korbanos he was Yotzei," is telling us that all the Tana'im agree to this. Even though Rebbi Yehudah requires that l'Chatchilah one should shave on the Shelamim, and Rebbi Eliezer requires that one should shave on the Chatas, nevertheless if he switched he did not lose the Mitzvah of Tiglachas, and even if he did it on the Olah he is Yotzei. The Chazon Ish writes that all this applies even according to Rebbi Eliezer.

b) However, the Chazon Ish then writes that Tosfos (47a, DH v'Nitma) says that according to Rebbi Eliezer he does not shave until he has brought all the Korbanos. (I add that we have been learning that the Rosh is going in the footsteps of this Tosfos.) I think that we learn two things from the way the Chazon Ish presents this: First, it seems that the Chazon Ish is telling us that the central opinion in the Sugya is not like Tosfos (47a). The simple way of learning the Sugya is that Rebbi Eliezer agrees that it is sufficient to shave on one of the Korbanos. Tosfos (47a) and the Rosh are a more Mechudash opinion -- that one needs all three Korbanos for the Gilu'ach to work. Second, the Chazon Ish makes no attempt to have Rebbi Eliezer agree with the Mishnah (45a) that he is Yotzei if he shaved on one of the Korbanos. The Chazon Ish seems to understand that it is obvious that Rebbi Eliezer cannot agree with this. Possibly, this does not conform with the Tosfos Yom Tov that I cited earlier, that this Mishnah applies according to everyone, and it also does not seem to be like the Rashash that I cited earlier, who says that we have not found that Rebbi Eliezer disagrees with the Mishnah on 47a.

c) To answer your original question, Sender (where do we see that we need all three Korbanos for Gilu'ach), the Chazon Ish writes that Rebbi Eliezer knew this from logic, Sevara. Tosfos learns that since Rebbi Eliezer said on 46a that the Nazir may not drink wine until he has done all the Ma'asim, it is logical to say that if he is not yet allowed to drink wine he is also not yet allowed to shave. When the Torah says that the Tiglachas is a Mitzvah, this only applies when -- after the Tiglachas of the Mitzvah -- he now becomes permitted to shave whenever he likes. It is not logical to say that after the Mitzvah-shaving, he still must wait if he wants to shave again. Therefore, since Rebbi Eliezer says that he needs all three Korbanos in order to be allowed to drink wine, it is a Sevara that the Tiglachas of the Mitzvah cannot be done before he has brought all three Korbanos.

Kol Tuv,

Dovid Bloom