How would one pronounce the following words:

boys’ bags
boy’s bags
Tess’ bags

There is no difference in pronunciation between BOYS’ bags and BOY’s bags. The difference is in the written forms. The singular possessive has the apostrophe before the s; the plural has the apostrophe after the s. The final sound for both is [z], and the word is pronounced [boyz].

Rules for the pronunciation of possessives of nouns are the same as those for noun plurals and for verbs in the third person singular, present tense. The pronunciation of the ending depends on the final sound of the noun or verb.

The possessives of nouns (except for the special ones below) ending in voiceless consonants, e.g. [p], [t], and [k], end in [s], while the possessives of nouns ending in the voiced consonants, e.g. [ b], [d], and [g] are pronounced as [z].

True phonetic transcriptions being impossible, I will approximate the sounds of the examples.

I. If the noun ends in one of six special consonants (sibilants), the ending is [iz]:

[s] Tess’s [tesiz]
[z] Rose’s [rowziz]
[ch] Butch’s [buhtchiz]
[j] Midge’s [midjiz]
[sh] Rush’s [rushiz]
[zh] the garage’s [history] (final [zh] is very rare with proper names)

II. Nouns ending in vowels and voiced consonants add [z]:

Fay’s [fayz]
Hugh’s [hyooz]
Bob’s [bahbz]
Ted’s [tehdz]
Meg’s [mehgz]
Sam’s [saemz]
Dean’s [deenz]
Claire’s [klehrz]
Paul’s [pahlz]
Bing’s [bingz]

III. Nouns ending in voiceless consonants add [s]:

Hope’s [hohps]
Pat’s [paets]
Clark’s [klarks]
Ruth’s [rooths]