Is it obligatory to fast three days for someone who intended to enter into ihram on the plane when passing the miqat, then entered Makkah without ihram, and then entered into ihram from there and offered a fidyah?

If you intend to perform Hajj at the Miqat, this constitutes the state of Ihram, and no verbal utterance is required, as its place is the heart. If you remain in your ordinary clothes after intending Ihram, you must repent and offer a fidyah (expiation) by sacrificing a sheep, or fasting for three days, or feeding six needy persons. The fidyah must be offered within the Haram and distributed among its poor. Scholars differ on the permissibility of paying the monetary value of the fidyah; the Hanafis permit it. If you pay its value in Mecca, then there is no blame upon you.