I've never known the term 'cash-cow' to be used in the context of dairy-farming. (That doesn't mean you can't do it of course; but you're wise to use the word 'literally'.)
1) "The farmers are living off his(? their?) cash cows, literally" OK, but 'the 'literally' feels to me as though it applies to the verb.
2) "Those cows really
putgive the wordexpression 'cash cows' inits literal meaning"
3) "The farmers are living out the
word[the meaning of] the expression cash cows literally"
(Despite what I said about 'literally' I think you can do without it after you've said 'living out'.
- For Teachers