Hi! Thanks for the question!
1. It is not safe to go for the property as you are not a party to the agreement between A,B and C and hence the clause may not be in your favour and there may be limitations that might not be comfortable for you in future. So you should first know the agreement thoroughly and then deal in.
2. To be on the safe side, as I said above, first have the agreement sorted on the initial stage itself. Once it is registered and sorted between A,B and C, you can buy the property without any risk.
You will have to keep in mind a few things if you directly go for registration of the road in your name. Firstly, that as the law of easement is involved, how long has it been since the passage is in use and in what fashion because if it has been in use in a free and regular manner then C would not be allowed by the court to deny the easement.Secondly the clauses mentioned in the agreement between A, B and C. When you are sure that that agreement can incorporate and serve your needs in future, only then go for it.
3. I have mailed you an examplary document with the format of the easement documents and various clauses that you should include in the agreement to make it comprehensive and exhaustive.(On the email ID - manicbe1982@gmail.com)
Hope this will help! Please feel free to ask any further questions! Good Luck!