Praise be to Allah.
The days of Tashreeq are the three days following Eid al-Adha, namely the 11th, 12th and 13th of Dhu’l-Hijjah. It is haraam to fast on these days.
The Prophet (peace and blessings of Allaah be upon him) said: “The days of Tashreeq are the days of eating and drinking.” Narrated by Muslim, 1141, from the hadeeth of Nubayshah al-Hadhali.
And he said: “The day of ‘Arafah, the day of Sacrifice (al-Nahr) and the days of Tashreeq are our Eid, us Muslims, and they are days of eating and drinking. Narrated by al-Nasaa’i , 3004; al-Tirmidhi, 773; Abu Dawood, 2419 – from the hadeeth of ‘Uqbah ibn ‘Aamir. Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
The Prophet (peace and blessings of Allaah be upon him) did not grant a concession allowing anyone to fast on these three days except for the pilgrims performing tamattu’ or qiraan who did not have sacrificial animal with them. Al-Bukhaari (1998) narrated that ‘Aa’ishah and Ibn ‘Umar (may Allaah be pleased with them) said: No one was allowed to fast on the days of Tashreeq except the (pilgrim) who did not have a sacrificial animal.
Hence the majority of scholars said that it is not permissible to fast on these days, regardless of whether that is a naafil fast, making up a missed fast or fasting in fulfillment of a vow. They said that a fast was invalid if it was observed during this time.
The most correct view is that of the majority, and there are no exceptions except in the case of the pilgrim who does not have a sacrificial animal.
Shaykh Ibn Baaz (may Allaah have mercy on him) said:
Similar fasts should not be observed on the day of Eid al-Adha or the days of Tashreeq, because the Messenger of Allaah (peace and blessings of Allaah be upon him) forbade that. But there is evidence that it is permissible to fast on the days of Tashreeq for the pilgrim performing tamattu’ or qiraan Hajj in lieu of offering the sacrifice, especially those who are unable to offer the sacrifice. But fasting them on a voluntary basis or for any other reason is not permissible, as in the case of Eid.”
Quoted from Fatawa Ramadaan, compiled by Ashraf ‘Abd al-Maqsood, p. 716
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
“It is permissible for the pilgrim performing tamattu’ or qiraan, if he cannot find a sacrificial animal, to fast these three days so that he can fast them during the Hajj season. But apart from that it is not permissible to fast these days; even if a person is fasting two consecutive months, he must not fast on the day of Eid and the three days that follow it, then he should resume his fasting.”
Fataawa Ramadaan, p. 727
Based on this, the fasts that you observed on these days to make up for missed Ramadaan fasts, are not valid, and you must repeat them.
When making up missed Ramadaan fasts, the days need not be consecutive, so you can make up the missed fasts on consecutive days or on separate days.
See also Question no. 21697
And Allaah knows best.