One has  to distinguish between two different kinds of books that might be called "Sheloshah sefarim niftahim."
There's the kind where the three books are all by the same author, and the kind where they are by different authors (or at least by more than one author).
                                                                    ,
If they're all by the same author,  rule 25.9A applies:  "Three or more  works ... in one form if the person created works in one form only"  In that case, one needs the uniform title "Selections. Date."  For that kind of work, one needs  505 but don't have to trace name-titles or the titles of the included works.

If they're by different authors, the main entry is the collective title and there  are all those 7XXs that are mentioned per 21.7B1: "enter a work  (consisting of  works  by different persons or bodies) under  its  title if  it  has  a collective title" and later "if such an item  includes two  or  three works,  make name-title added entries for  each  of them."  Properly speaking, the  names  of  the individual works  are  not subtitles ($b), since they don't relate to  the work as  a whole;
they  belong  in  a contents note  (505)  which  will justify the 700s.

As for the cover and spine titles that mention all the individual titles, those  are treated the same way for both kinds.
One makes a cover title, but it only has to be for the "cover title proper," and since we now put the $b after the first title in        such  cases, your "cover title" 246 will give only the first of the titles.
 

Example

100 10 Author.
240 10 Berit avot
245 10 Sefer Berit avot ; Pidyon nefesh.
700 12 Author.$tPidyon nefesh.
740 02 Pidyon nefesh.

We wouldn't mention the other titles at  all  in  the bib record,  but we would make NARs for both of them, as follows:

100 10 Author.$tBerit avot
400 10 Author.$tSefer Sharvit ha-zahav

100 10 Author.$tPidyon nefesh
400 10 Author.$tSefer Sha'are pedut

We won't add refs for  the variant titles without Sefer  (since they  don't appear  that  way  and  they  aren't "uniform"
titles), but one  can  do  so.  "Berit  avot" and "Pidyon nefesh" get to  be the uniform titles by virtue of appearing in the chief source.