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Of American-English origin, the phrase from shirtsleeves to shirtsleeves in three generations, and its variants, mean: wealth gained in one generation of a family will be lost by the third generation.

This phrase occurs, for example, in the following from How a dream of becoming rich can turn into a nightmare, by Eleanor K. Szymanski, published in The Times (Trenton, New Jersey, USA) of Sunday 7^th July 2024 [page A9, column 3]:

There’s an old adage that says it all: “Shirtsleeves to shirtsleeves in three generations.”
Essentially, this sums up the difficulties associated with wealth passing suddenly and sometimes unexpectedly, to the uninitiated.
Super wealthy folks such as Warren Buffet ruminate publicly about their concerns for the effect on and wellbeing of family members who might inherit their huge wealth.
But many of the not-so-super-wealthy also deal with windfalls.
It’s not the size of the windfall but the influence it will have on the recipient—most of which is usually positive but much of it causing major adjustments.

In reference to a hard-working man wearing a shirt with nothing over it, the phrase from shirtsleeves to shirtsleeves in three generations expresses the idea that the first generation of a family will acquire wealth through hard work, the second will live an affluent life but lack a strong work ethic, and, as a result, by the third generation, the family will be reduced to their original circumstances.
—British-English synonym: from clogs to clogs in three generations.

These are, in chronological order, the earliest occurrences of the phrase from shirtsleeves to shirtsleeves in three generations and variants that I have found:
—Note: this phrase has sometimes been attributed to the Scottish-born U.S. manufacturer and philanthropist Andrew Carnegie (1835-1919), but there is no evidence that he coined it:

1-: From a portrait of the U.S. essayist and novelist Charles Dudley Warner (1829-1900), by the U.S. poet, critic and essayist Edmund Clarence Stedman (1833-1908), published in Appletons’ Journal of Literature, Science, and Art (New York City, New York, USA) of Saturday 27^th June 1874 [page 802, column 3]:

Turn a university-man into the world of England, where every place is marked “taken,” and what a hapless future awaits him! Possibly that of an usher in a charity-school, more defenseless than a hedger and ditcher. If he has grit and brains he may come to something, yet the chances are wofully against him. In America, where there are “but three generations from shirt-sleeves to shirt-sleeves,” the whole of a son’s fortune is well spent upon his training, though he must go out into life with the tastes of a gentleman, and no income to sustain them.

2-: From The Rapid Progress of Communism, by the U.S. economist Edward Atkinson (1827-1905), published in The Atlantic Monthly: A Magazine of Literature, Science, Art, and Politics (Boston, Massachusetts, USA) of June 1882 [page 834, column 1]:

It is an old saying that “it usually takes but three generations from shirt-sleeves to shirt-sleeves.” This was true in olden time in this country. Wealth seldom stayed in one family more than three generations. In these later days it may stay a little longer, because we have in some measure provided better for the education of the sons of rich men than we did formerly, and have in greater degree removed their disabilities, and rendered them capable of making such use of the property devised to them as may enable them to keep it. The power of rich men to impose riches upon their descendants beyond their grandchildren has been taken away by law; and it may happen, as the dangers of inflicting the disabilities of wealth upon grandchildren become better understood, that the law will forbid wills which place it out of the power of the parents of the child to divert bequests which are so often only a misfortune.

3-: From the Franklin Repository (Chambersburg, Pennsylvania, USA) of Tuesday 17^th April 1883 [page 2, columns 2 & 3]—reprinted from the Philadelphia Times:

FROM SHIRT-SLEEVES TO SHIRT-SLEEVES.

It is an old and true saying that “it’s only three generations from shirt-sleeves to shirt-sleeves,” and the now common reports of frauds, defalcations, embezzlements and flights of the sons of men who started in their shirt-sleeves and rose to fortune, prove that if there is any departure from the old rule in our day, it is in limiting the endurance of fortune in American families.
[…]
[…] The fathers who started in their shirt-sleeves and built up character and fortune by patient industry forget that they began as shop-sweeps and rose to the position of proprietors over the idle sons of their early masters, and their sons go the way of the other sons who should have been the honored successors of their fathers. The common rule now is for shirt-sleeves to win over the luxuriantly reared sons of our business men, and gain position and fortune only to be squandered by their sons in turn as they go back to shirt-sleeves in hopeless disgrace. […]
[…]
It is the inexorable law of the Almighty that man shall earn his bread by the sweat of his brow, and it is terrible in its revenges for all violations. There is no usefulness, no happiness, no prosperity in idleness, and he who rejects industry as an unworthy pursuit seeks unrest and readily drifts into misfortune and crime. The place for every boy to begin life is in his shop or factory or field and thus master a calling that will give him physical vigor and honest aims. The shirt-sleeve must be the welcome garb of the boy, if it would not be made his garb in shame in later years, when the best days of life are spent in worse than waste; and the kitchen must be the early school room of the girl, if she would not be the miserable dependent of servants and wholly unfitted for a mother’s supreme duties. From shirt-sleeves to shirt-sleeves—such is the immutable law of life, and it is only a question whether they shall be welcomed in honor, or come unbidden in shame.