You are not a kaafir for not praying Jummah three times in a row. The Hadith you are talking about says that Allah (SWT) will put a seal over his heart, which basically means that he will become heedless to the guidance of Allah (SWT), may Allah save us from that.
It was narrated on the authority of Abu Hurayrah and ‘Abdullaah Ibn ‘Umar may Allaah be pleased with them that the Prophet , threatened the one who leaves the Friday prayer that Allaah The Exalted would close his heart and thus, he would become one of the heedless. [Muslim]
It was narrated on the authority of Abu Al-Ja‘d Adh-Dhamri may Allaah be pleased with him that the Messenger of Allaah , said that whoever misses three consecutive Friday prayers out of negligence will have a seal put over his heart by Allaah The Almighty to prevent it from receiving guidance. [At-Tirmithi] [source]1
Attending jummah prayer is obligatory and one cannot leave it unless he has Shareah approved reasons, which don't include going to school, studying, etc. [https://islamqa.info/en/36530]. Allah (SWT) says in the Quran:
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا نُودِيَ لِلصَّلَاةِ مِن يَوْمِ الْجُمُعَةِ فَاسْعَوْا إِلَىٰ ذِكْرِ اللَّهِ وَذَرُوا الْبَيْعَ ۚ ذَٰلِكُمْ خَيْرٌ لَّكُمْ إِن كُنتُمْ تَعْلَمُونَ
O you who have believed, when [the adhan] is called for the prayer on the day of Jumu'ah [Friday], then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew. 62:09
I'd recommend that you talk to your professor and explain the situation to him. I am sure he will give you permission. If not, you should obey Allah (SWT) as obeying the Creator takes precedence over all things.