There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the word: circus'
Add an apostrophe s ('s) after the existing s at the end of the word: circus's
Examples:
The circus' star was the high wire walker.
The circus's star was the high wire walker.
The plural form for the noun circus is sircuses. The plural possessive form is circuses'.
We visited all the different circuses to decide which one was better.
Circus's
The singular possessive form is heart's; the plural possessive form is hearts'.
The plural form is founders. The plural possessive is founders'.
The possessive form is the circus monkey'scoat.
The plural form for the noun sergeant is sergeants; the plural possessive form is sergeants'.
The singular possessive form for the noun knife is knife's.The plural form is knives; the plural possessive form is knives'.
The noun 'sister' is not plural and is not possessive.The plural form is sisters.The possessive form is sister's.The plural possessive form is sisters'.
The singular form is goose; the singular possessive form is goose's.The plural form is geese; the plural possessive form is geese's.
The plural possessive form of "cherubim" is "cherubim's."
The plural possessive form of "reply" is "replies'".
The plural form is valleys. The plural possessive is valleys'.
The plural form is squirrels. The plural possessive is squirrels'.
The plural form is cockroaches; the plural possessive form is cockroaches'.