I am going to prepare a 12v battery using 3 18650 Lithium Ion batteries in series. As i am new to these batteries, i read some articles about it. But I'm not clear in charging it. Suggest me the correct circuit for charging 3S 18650 battery pack.
You need to manage the charge balancing between the individual cells. Different cells will have (slightly) different capacities, and different self-discharge rates. Unless managed, the state of charge of all cells will not remain matched and will drift apart. If you do not manage this, ultimately, when charging the pack, some cells will become dangerously overcharged, while others will not be fully charged. Similarly, on discharge, some may become over discharged.
It is imperative to use a battery management system to avoid these issues. This will maintain the same state of charge on all cells in the pack. As an alternative, consider connecting the cells in parallel, and using a DC/DC converter to deliver the 12 V you need.
You should consider what you are powering and the voltage range it requires. A 3S battery will be only 9V when drained (3v) whereas a 4S lithium will be 12v. A fully charged cell is 4.2 volts so if you're using 3 in series you should charge to 12.6 volts
Voltage sag or drop should also be considered. Some cells will drop considerably especially if you're using repurposed laptop cells. You should also have a BMS (battery management system) to balance the individual cells. The BMS can also protect against over discharge and over current.