They don't.
The Ganges flow into the bay of Bengal wheras the Narmada and Tapi flows into the Arabian Sea (Gulf of Cambey).
Neither the Narmada or Tapi/Tapti Rivers meet to form the Ganges. See the Related Link below to see which rivers meet to form the Ganges.
Ganga river is fed by many perennial rivers and steams in its higher course. Mandakini originates from a glacier near Kedarnath in Uttarakhand (a state in India). Mandakini joins Alaknanda at Rudraprayag which in turns proceed to meet Bhagirathi river in Devaprayag to form river Ganga. As being one of ganga's tributaries, Mandakini is a synonym for river Ganga.
Bhagirathi and Alaknanda are two most important tributaries of Ganga. Both tributaries meet at Devprayag to form Ganga.
triveni sangam
the main two headstreams are alakananda and devaprayag
allahbad
There are 7 sacred or special rivers:Ganga or River Ganges being the most important.River Yamuna / JamunaRiver GodavariRiver SaraswathiRiver NarmadaRiver SindhuRiver Kaveri or CauveryIn-fact, there is a Shloka that Hindus recite during they take bath.Gange cha Yamune chaiva Godavari Saraswathi,Narmada Sindhu Kaveri Jale asmin sannidhim kuru.It means:Oh , holy river Ganga,Oh, Holy river Yamuna,Oh , Holy river Godavari,Oh, Holy river Saraswathi,Oh , Holy river Narmada,Oh holy river Sindhu,And holy river Kaveri,I pray you to enter this water, which is meant for my bath.
Ganga-Yamuna
Both rivers are in Ireland but they do not meet.
A river
St. Louis is where the Missouri River and the Mississippi River meet.
Tigris river and the europhease river meet at mesopotamia