Nahi, Nahe & Nahin are the same word " Nahi ". While writing Urdu in roman, you find different spelling of Nahi. "Nahi" is the correct spelling. "Nahi" means " No ". Example : Ma aaj school nahi jaoon gi . ( I will not go to school today)

as correctly said they are same words نہیں . In hindi correct spelling is nahin because there is nasal sound in 'hee' part which should be accounted, like we say in 'hain' it is hai but with a nasal sound in the end. another example 'hoon'. In urdu that nasal sound in the end is denoted by a 'noon' (ن)without a dot(i.e. ں) .