Question
What is the correct way of saying Nabi in the plural form?  Taking into account the rasm Alif Nuun Bey.... some people pronounce it is Anbiya using the first 3 Rasm alphabet whereas others pronounce it Ambiya.  Is there a technicality involved using Tajweed or this an error?

Answer
There are actually two forms of pluralizing the word , which can have a dhammah as the last vowel, or a fathah, or a kasrah, depending on its grammatical place in the sentence.  The first form of pluralizing is a regular form, which is , or ; and the use of one or the other is dependant on its grammatical place in the sentence. 
The other form, which is irregular, is , and this is the word referred to in the question.  There is a saakinah followed by a , and in taweed rules, when this case exists, the saakinah is changed into a and the is hidden by closing the lips on the meem, holding it for the length of time for the complete ghunnah, then the lips are separated while pronouncing the .  So one can spell it in transliteration into English letters using the written letters, al-anbiyaa', or the correct pronunciation of the letters, al-ambiyaa', whichever one prefers. 

Please also see: http://www.abouttajweed.com/the-noon-saakinah-and-tanween/changing-iqlaab-rule-of-the-noon-saakinah-and-ta.html