Here is what is meant by ummi in the different verses that you mentioned...
But before that there's a point I want to clarify about how we, muslims, read the Quran, we should understand what's meant by each word, what were the circumstances for each verse when it was revealed, and why it was revealed in what's known in arabic as (asbab-al-nouzoul).
Back to UMMI:
Sura 2:78
78. And there are among them (Jews) `Ummyyun (unlettered) people, who know not the Book, but they trust upon Amani (false desires) and they but guess.) (79. Then woe to those who write the book with their own hands and then say, "This is from Allah,'' to purchase with it a little price! Woe to them for what their hands have written and woe to them for that they earn thereby.)
The Meaning of `Ummi>>>Tafsir Ibn-Kathir
(And there are among them Ummyyun people) meaning, among the People of the Book, as Mujahid stated. Ummyyun, is plural for Ummi, that is, a person who does not write, as Abu Al-`Aliyah, Ar-Rabi`, Qatadah, Ibrahim An-Nakha`i and others said.This meaning is clarified by Allah's statement,(Who know not the Book) meaning, are they not aware of what is in it.
Ummi was one of the descriptions of the Prophet because he was unlettered. For instance, Allah said,Neither did you (O Muhammad ) read any book before it (this Qur'an) nor did you write any book (whatsoever) with your right hand. In that case, indeed, the followers of falsehood might have doubted)
Allah also said, He it is Who sent among the Ummiyyin ones a Messenger (Muhammad ) from among themselves) (62:2).
Sura 3:30>>>>i THINK THE VERSE YOU'VE MENTIONED ISN'T IN SURA 3, PLEASE LOOK AGAIN.
Sura 3:75
75. Among the People of the Scripture is he who, if entrusted with a Qintar (a great amount of wealth), will readily pay it back; and among them there is he who, if entrusted with a single silver coin, will not repay it unless you constantly stand demanding, because they say: "There is no blame on us to betray and take the properties of the illiterates (Arabs).'' But they tell a lie against Allah while they know it.) (76. Yes, whoever fulfills his pledge and fears Allah much; verily, then Allah loves the Muttaqin (the pious).)
Allah states that there are deceitful people among the Jews. He also warns the faithful against being deceived by them, because some of them,
(if entrusted with a Qintar (a great amount)) of money,(will readily pay it back This Ayah indicates that this type would likewise give what is less than a Qintar, as is obvious. However,(and among them there is he who, if entrusted with a single silver coin, will not repay it unless you constantly stand demanding,) and insisting on acquiring your rightful property. If this is what he would do with one Dinar, then what about what is more than a Dinar We mentioned the meaning of Qintar in the beginning of this Surah, while the value of Dinar is well known. Allah's statement,because they say: "There is no blame on us to betray and take the properties of the illiterates (Arabs).'') means, what made them reject the truth (or what they owed) is that they said, "There is no harm in our religion if we eat up the property of the unlettered ones, the Arabs, for Allah has allowed it for us.'' Allah replied,(But they tell a lie against Allah while they know it.) for they invented this lie and word of misguidance. Rather, Allah would not allow this money for them unless they had a right to it.
Allah the Exalted said,(He it is Who sent among the unlettered ones a Messenger from among themselves,) the word `unlettered' here refers to the Arabs. Allah the Exalted said in another Ayah,
(And say to those who were given the Scripture and those who are illiterates: "Do you submit yourselves If they do, they are rightly guided; but if they turn away, your duty is only to convey the message; and Allah is All-Seer of (His) servants.) (3:20) Mentioning the unlettered ones in specific here does not mean that Muhammad was only sent to them, because the blessing to the Arabs is greater than that of other nations.
It's obvious that the verse in Isaiah refers to someone specific, and it is very similar to the first revelation of the Quran to prophet Muhammed, as it's explained here:
Imam Ahmad recorded that `A'ishah said: The first thing that began happening with the Messenger of Allah from the revelation was dreams that he would see in his sleep that would come true. He would not see any dream except that it would come true just like the (clearness of) the daybreak in the morning. Then seclusion became beloved to him. So, he used to go to the cave of Hira' and devote himself to worship there for a number of nights, and he would bring provisions for that. Then he would return to Khadijah and replenish his provisions for a similar number of nights. This continued until the revelation suddenly came to him while he was in the cave of Hira'. The angel came to him while he was in the cave and said, "Read!'' The Messenger of Allah said,(I replied: "I am not one who reads.) Then he said, "So he (the angel) seized me and pressed me until I could no longer bear it. Then he released me and said: `Read!' So I replied: `I am not one who reads.' So, he pressed me a second time until I could no longer bear it. Then he released me and saidRead in the Name of your Lord who has created.)
Is it a coincidence?