I checked three translations of Gita 2.56. None of the translators uses the term threefold miseries in translating the verse. However, Sri Sankaracharya talks about threefold miseries in his commentary.
Whose mind is not agitated in adversity, who is free from desire and
who is devoid of attachments, fear and anger - such a person is called
a sage of steady wisdom.
Gita 2.56 translated by Swami Tapasyananda
He whose mind is untroubled in the midst of sorrows and is free from
eager desire amid pleasures, he from whom passion, fear and rage have
passed away, he is called a sage of settled intelligence.
Gita 2.56 by Radhakrishnan
The silent sage is said to have stable wisdom when sufferings no
longer perturb his mind, when he ceases to cling to pleasures, and he
is rid of attachment, fear and wrath.
Gita 2.56 of Sri Sankaracharaya translated by Dr. A. G. Krishna Warrier
What is Tapatraya (three miseries)?
Adhibhautika, adhidaivika and adhyatmika are known as Tapatraya.
The three words respectively mean: pertaining to (a) the bhuta or
living beings; (b) the daiva or fate, unseen forces and gods; (c) the
body (and the mind).
Sorrow and suffering (duhkha, tapa) are inevitable in life. In fact,
they are a part of life. A knowledge of their origin, causes and even
categorisation helps one to minimise their effect if not eradicate
them. The Hindu religious works usually call them 'tapatraya', 'the
three miseries'. They are: adhyatmika, adhidaivika and adhibhautika.
The adhyatmika duhkha or tapa is that which is caused by bodily
suffering and mental anguish. Hereditary diseases like leprosy,
disabilities like blindness or lameness and diseases caused by the
violation of the rules of health and sanitation are classed under
this. The mental agony caused by worries and anxieties, attachment and
aversion, also comes under this group.
The adhidaivika duhkha or tapa is that which is caused by daiva. The
word daiva includes the power of time, nature and the unseen hand of
fate. Diseases caused by the changing seasons, misery caused by the
elemental forces like floods and fire, suffering caused by black magic
or gods who are displeased, natural tribulations due to hunger, thirst
and old-age belong to this group.
The adhibhautika duhkha or tapa is that which is caused by other
bhutas or living beings, like wild animals, snakes or enemies.
Some of these, like hereditary diseases or physical disabilities
cannot be got rid of. Hence they must be endured. Some like the
diseases caused by change of seasons or the machinations of enemies
can be countered by taking appropriate precautions. However, raising
the mind to the level of the spirit, thus transcending the limitations
imposed by the body-mind complex, is the best solution to offset the
effects of tapatraya.
A Concise Encyclopedia of Hinduism by Swami Harshananda