The Internal Revenue Code of 1954, all 1024 pages of it, was enacted in the course of a year, basically rebooting and rewriting the tax code, contains §1211 in in 68A Stat. 321:
(b) OTHER TAXPAYERS.—In the case of a taxpayer other
than a corporation, losses from sales or exchanges of
capital assets shall be allowed only to the extent of the
gains from such sales or ex-changes, plus the taxable
income of the taxpayer or $1,000, whichever is smaller. For
purposes of this subsection, taxable income shall be computed
without regard to gains or losses from sales or exchanges of
capital assets and without regard to the deductions provided in section 151 (relating to personal exemptions) or any
deduction in lieu thereof. If the taxpayer elects to pay
the optional tax imposed by section 3, "taxable income" as
used in this subsection shall be read as "adjusted gross
income".
This was rewritten in 1969, 83 Stat. 642, only 256 pages, to say
(1) IN GENERAL.—In the case of a taxpayer other than a
corporation, losses from sales or exchanges of capital assets
shall be allowed only to the extent of the gains from
such sales or exchanges, plus (if such losses exceed such
gains) whichever of the following is smallest: " (A) the taxable
income for the taxable year, "(B) $1,000, or "(C) the sum of—
"(i) the excess of the net short-term capital loss over the
net long-term capital gain, and " (ii) one-half of the excess of
the net long-term capital loss over the net short-term capital gain.
"(2) MARRIED INDIVIDUALS.—In the case of a husband or wife
who files a separate return, the amount specified in
paragraph (1) (B) shall be $500 in lieu of $1,000.
In 1976, Pub. L. 94-455, 1401(a,b) makes a further amendment, by replacing "$1,000" with "the applicable amount" and then defining the applicable amount for 1977 and for years thereafter:
(2) APPLICABLE AMOUNT.—For purposes of paragraph (1) (B),
the term 'applicable amount' means—
"(A) $2,000 in the case of any taxable year beginning in
1977; a n d
" (]3) $3,000 in the case of any taxable year beginning after
1977.
So it has been that way since 1978.