I would like to add on to r13 and 水巷孑蠻's answer.

0. Zhuxi's central theme here is the problem of annotating classics (經注之禍). Zhuxi was doubtful of Mencius's interpretation of *Shangshu* (see the bolded parts below). Consider the following somewhat better punctuated text with paragraphs before and after your excerpt.

   I also think the 陶弘景 and 唐子西 quotes are only examples to illustrate his disapproval of Mencius's interpretation. They are secondary, in a sense. 陶弘景 allures to 孟子 and 唐子西 allures to himself.

> 孟子曰：「盡信《書》，則不如無《書》。仁人無敵於天下，以至仁伐不仁，而何其血之流忤也？」曰：「紂一人惡邪？眾人惡邪？眾皆善而紂獨惡，則去紂久矣，不待周也。夫為天下逋逃主，萃淵藪，同之者可遽數邪？紂亡則逋逃者曷歸乎？其欲拒周者人可數邪？血流漂杵，未足多也。」或曰：「前徒倒戈，攻於後，以北。故荀卿曰殺者皆商人，非周人也。然則商人之不拒周，審矣。」曰：「如皆北也，焉用攻？」又曰：「甚哉：世人之好異也！孔子非吾師乎？眾言驩，千徑百道，倖存孔子，吾得以求其是。《虞》、《夏》、《商》、《周》之《書》出於孔子，其誰不知？ **孟子一言，人皆畔之。畔之不已，故今人之取孟子以斷《六經》矣。嗚呼，信孟子而不信經，是猶信他人而疑父母也。**」
>
> 余隱之曰：《魯語》曰：「俎豆之事，則嘗聞之矣。軍旅之事，未之學也。」孔子之意可見矣。客有問陶弘景注《易》與《本草》孰先，陶曰：「注《易》誤，不至殺人。注《本草》誤，則有不得其死者。」世以為知言。唐子西嘗曰： 「弘景知本草而未知經。注《本草》誤，其禍疾而小。注《六經》誤，其禍遲而大。」前世儒臣引經誤國，其禍至於伏屍百萬，流血千里。《武成》曰「血流漂杵」，武王以此自多之辭。當時倒戈攻後，殺傷固多，非止一處，豈至血流漂杵乎？孟子深慮戰國之君以此藉口，故曰「盡信《書》則不如無《書》」。而謂血流漂杵未足為多，豈示訓之意哉？經注之禍，正此類也。反以孟子為畔經，是亦惑矣。謂《虞》、《夏》、《商》、《周》之《書》出於孔子，人宜取信。《詩》非孔子之刪乎？《雲漢》之詩曰：「周余黎民，靡有孑遺。」信斯言也，則是周無遺民也。請以此說為證。

Regarding your questions:

1. 自多 means 'to be full of oneself; to boast' (自滿；自誇), where 多 is the verb. 血流漂杵，武王以此自多之辭 is an abbreviated statement: 血流漂杵(者)，武王以此自多之辭(也). It translates as: (to Mencius) 血流漂杵 is a phrase by which King Wu of Chou used to self-boast.

2. 止通只. As explained by the rest.

3. 多 is exactly the same meaning as in the case of 1. As a side note, the online sources I searched did not show 豈示訓之至哉 but 豈示訓之意哉, and I managed to find 《古今圖書集成》 to have used 意 as well (see [here][2]). Anyhow I believe the hidden subject is Mencius: (孟子)謂血流漂杵未足為多，豈(孟子)示訓之至哉, because with contextual clues I do not think 血流漂杵未足為多 is Zhuxi's claim, but rather Zhuxi's quote of Mencius's claim.

    Translating (孟子)謂血流漂杵未足為多: ‘Mencius considered '血流漂杵' not as the most self-boastful [meaning there are even worse instances of the King's self-boasting according to Mencius]’. It is not impossible that Zhuxi was mocking Mencius here, given what we saw in 0.


  [1]: http://www.guoxue123.com/other/xuean/songyuanxuean1/004.htm
  [2]: https://zh.wikisource.org/wiki/Page:Gujin_Tushu_Jicheng,_Volume_578_(1700-1725).djvu/111
  [3]: https://wyw.hwxnet.com/view/hwxE5hwxB2hwx82.html
  [4]: https://i.sstatic.net/TFMWom.png