There are no high speed trains between the three cities. From Chengdu to Chongqing, bullet trains are available, and it cost around CNY67 for one hard seat. But there is only one ordinary passenger train on Nanchong-Chongqing route. It takes around 8hrs. For more details, u could check in the search form on China Trains(https://www.travelchinaguide.com/china-trains/) .