Can I prepare the three day notice myself?

Yes, but you shouldn’t.  A three day notice is part of the pleadings in your eviction lawsuit.  Really, it should be done by a lawyer.  You need the lawyer to file the eviction lawsuit anyway, why not have the lawyer prepare the notice also?

The tenant has not paid in two months, why do I have to give the tenant a three day notice?

Because the law says so.  More importantly, the notice is part of the pleadings in your eviction lawsuit.  The notice needs to be prepared carefully using the exact spelling of the tenant’s name (not his/her nickname).  The notice must contain the amount of the unpaid rent (not every little nit picky thing you think is owing).  The notice must contain an address where the rent can be delivered.  Rent can not be delivered to a P.O. Box.