丙

08:46 May 5, 2012 

甲, 乙 or 丙 is a part of the title of the respective police document. For example, a statement from a suspect is recorded in the 供述調書（甲）and a witness statement in the 供述調書（乙）. See the Article 9 in the following link (http://www.moj.go.jp/keiji1/keiji_keiji16.html). The 供述調書（甲）is Form 16（様式第16号）and the 供述調書（乙）is Form 17（様式第17号）. As any other forms, the 供述調書 and the 弁解録取書 are used for different subjects of investigation (or situations) and 甲, 乙 or 丙 is included in the title to specify the subject. These characters are more or less proper nouns/names. They are not used for numbering documents or placing them in order of priority. One way to deal with these characters in your translation will be to put them as Kou, Otsu and Hei, just like when a Japanese person’s name is translated.

08:29 May 4, 2012 

They are situated solely on top of the pages within brackets. There's nothing used with the characters. Just the characters in brackets on top of the pages. 甲 on the first page, 丙 on the rest & 乙 on the first page, 丙 on the rest.

00:55 May 4, 2012 

Both 供述調書 （a.k.a. 供述録取書）and 弁解録取書 have normally little to do with a plaintiff （原告）, a defendant（被告）, and a third party（第三当事者、訴訟参加人 or 共同被告人）They are police records developed as a result of an investigation. Statements by a suspect, the arrested, the accused or a witness are recorded by the police in these documents.
If you could tell me with what words each of 甲, 乙 and 丙 appears in the source text, I would be able to tell what they mean.