1. The words : ان الله ثالث ثلاثة in verse 72, translated here literally as 'Allah is the third of the three,' refers to Sayyidna Masih (Jesus Christ), Ruhul-Quds (The Holy Spirit) and Allah, or to Masih, Maryam (Mary) and Allah - with all three taken as God (Refuge with Allah). thus , Allah becomes a one-third partner with them, then all three of them become one, and that one is three. This unity in trinity, with three persons in one godhead, is the common belief of Christians. They explain this belief which is supported by neither reason nor revelation with a language that is kept circuitous and ambiguous. When not understood, they declare it to be reality beyond comprehension. (Shabbir Ahmad Usmani - Notes).
2. In verse 75, by saying: قد خلت من قبله الرسل (There have been messengers before him), the assigning of godhood to Sayyidna Masih عليه السلام has been refuted. Prophets came to this world. They completed their mission. Then, they were gone. They were not eternal which is the mark of godhood. Similarly, Sayyidna Masih, may peace be upon him (being human like them) was not eternal. So, he cannot be what God is.
A little reflection will show that everyone who needs to eat and drink almost depends on everything in the world. We cannot say that the earth, the air, the water, the sun and the animals are what we do not need. Look into your own self. There is that grain of food outside you, then begins its journey from the entry into the stomach to the next destination of its digestion. Think of all those factors involved and things required directly and indirectly in this complex procedure. Then, there will be a chain of effects generated through eating and no one can tell precisely how far will they go. So, by pointing out that Sayyidna Masih and his pious mother used to eat, the Holy Qur'an has referred to the endless chain of needs it entails. The argument, thus runs that Masih and Maryam, may peace be upnrl them both, were not free of the need for eating and drinking which is proved by observation and authentic narrations (not denied by even Christians). And anyone who is not free from the need of eating and drinking cannot be free from anything in this world. How then, a human person, who like all human beings is not free of the need to depend on the chain of causation for survival, could become God? This is a strong and clear proof which can be understood by the educated and the uneducated alike that is, eating and drinking is contrary to godhood. Though, not eating too is not a proof in favour of godhood, otherwise all angels would become gods! (Refuge with Allah) (Tafsir Usmhni).
3. Was Sayyidah Maryam a prophet or saint? This is a debated issue. In the present verse (75), the complimentary use of the word "Siddiqah" (truthful) obviously seems to indicate that she was a godly person, not a Nabiyy ( prophet) because on a complimentary occasion, what is mentioned is the higher rank. If she had the station of prophethood, the word used for her at this place would have been "Nabiyyah"- but, the word used here is "Siddiqah" which is the station of sainthood or godliness (abridged from R i h al-Ma'ani).
According to the majority of Muslim scholars, the station of Nubuwwah (prophethood) has never appeared among women. This mission has been particular with men:
وَمَا أَرْسَلْنَا مِن قَبْلِكَ إِلَّا رِجَالًا نُّوحِي إِلَيْهِم مِّنْ أَهْلِ الْقُرَىٰ
And We have not sent any (one) before you but men to whom We revealed from among the peoples of'the towns Surah Yusuf, 12 : 109. (Tafsir-Usmhni).
Verses 77 - 81
قُلْ يَا أَهْلَ الْكِتَابِ لَا تَغْلُوا فِي دِينِكُمْ غَيْرَ الْحَقِّ وَلَا تَتَّبِعُوا أَهْوَاءَ قَوْمٍ قَدْ ضَلُّوا مِن قَبْلُ وَأَضَلُّوا كَثِيرًا وَضَلُّوا عَن سَوَاءِ السَّبِيلِ
لُعِنَ الَّذِينَ كَفَرُوا مِن بَنِي إِسْرَائِيلَ عَلَىٰ لِسَانِ دَاوُودَ وَعِيسَى ابْنِ مَرْيَمَ ۚ ذَٰلِكَ بِمَا عَصَوا وَّكَانُوا يَعْتَدُونَ
كَانُوا لَا يَتَنَاهَوْنَ عَن مُّنكَرٍ فَعَلُوهُ ۚ لَبِئْسَ مَا كَانُوا يَفْعَلُونَ
تَرَىٰ كَثِيرًا مِّنْهُمْ يَتَوَلَّوْنَ الَّذِينَ كَفَرُوا ۚ لَبِئْسَ مَا قَدَّمَتْ لَهُمْ أَنفُسُهُمْ أَن سَخِطَ اللَّهُ عَلَيْهِمْ وَفِي الْعَذَابِ هُمْ خَالِدُونَ
وَلَوْ كَانُوا يُؤْمِنُونَ بِاللَّهِ وَالنَّبِيِّ وَمَا أُنزِلَ إِلَيْهِ مَا اتَّخَذُوهُمْ أَوْلِيَاءَ وَلَـٰكِنَّ كَثِيرًا مِّنْهُمْ فَاسِقُونَ
Say, "O people of the Book, be not excessive in your faith unjustly, and do not follow the desires of a people who already went astray, misled many and lost the right path. 
On those who disbelieve from among the children of Isra:'il was a curse pronounced by Dawud and 'Is; son of Maryam. All this because they disobeyed and used to cross the limits.  They did not forbid each other from an evil they committed. Indeed, evil is what they have been doing. 
You see many of them take the disbelievers as friends. Indeed, evil is what their own selves have sent ahead for themselves as Allah is angry with them, and they are to remain under punishment for ever.  And if they have been believing in Allah, in the prophet and in what had been sent down to him, they would have not taken them as friends, but many of them are sinners. 
The Crookedness of Bani Isra'il :Yet Another Aspect
It will be recalled that, in previous verses, mentioned there was the contumacy of Bani Isra'il along with their tyranny and injustice with reference to what they did to their benefactors, the prophets sent by Allah who had come to show them the way to succeed in this life and in the life to come. But, they mistreated them, by falsifying some and killing others: فَرِيقًا كَذَّبُوا وَفَرِيقًا يَقْتُلُونَ (72)