The M"B brings in the beginning of Siman 47 from the Shagas Aryeh (Siman 24-25) that if one is in sofek (doubt) if they said Bircas HaTorah then they should go back and say the bracha however only the bracha of "asher bachar banu" (And not all three brachas that we say lichatchila.) This goes in accordance with the Rishonim that hold Bircas HaTorah is min haTorah.
However why to say only one? I realize the Shagas Aryeh says because if you say one you have fulfilled your obligation from the Torah and the other 2 brachos are then considered to be only d'rabbanan and then we wouldn't say those because of a doubt. However we see by Bircas HaMazon that if one is in sofek if they said it or not (and ate to the point of being satiated) then they must now say it (Shulchan Aruch Orach Chaim 184:4.) The M"B there in Note 13 says that one should even say the 4th bracha which is only d'rabbanan in order that one should not be m'zalzel (make light of?) the brachas of Bircas HaMazon.
Why according to the Shagas Aryeh and what he says about Bircas HaTorah would this not apply? Even if with one bracha you are yotzee m'doraisa (making the other 2 then d'rabbanan.) However why doesnt' this idea of not wanting to be m'zalzel not apply in this case of Bircas HaTorah.
(Note: I'm asking this only according to the Shagas Aryeh because I don't know what the Rishonim that hold Bircas HaTorah is d'oraisa would say in this case. Perhaps they would hold that one should say all 3 brachas again, maybe not? If someone has a proof to this either way please edit or comment. As well perhaps this can only be asked on the Shagas Aryeh if the whole idea of saying the 4th bracha as well is a "later" idea. THe M"B brings the makor for not being "m'zalzel" from "Achronim". However I don't know what the earlier sources say. Perhaps someone could help with this as well.)
