grammatical number - Plural of an initialism written in lowercase with abbreviation full stops

Although similar questions have been already posed and answered here, I still have this doubt: in the case of initialisms written in lowercase (and possibly having full stops after each letter), how should we write their plurals? Should we be using an apostrophe before the 's'; no apostrophe; no 's' at all...?

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EXAMPLE 1: The term random variable is usually abbreviated as r.v. So, several random variables could be referred to as r.v.'s?? If not, how?

EXAMPLE 2: Cumulative distribution function is usually abbreviated as c.d.f.; or even cdf, very often.

EXAMPLE 3: Very famous in Maths: greatest common divisor is usually abbreviated as gcd.

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This question differs from previous ones in that I am interested in the plural of initialisms / acronyms written in lowercase.