I would like to emphasize something Ven wrote earlier.
When a flashlight uses multiple Li-ion 18650 batteries, best practice is to buy all batteries at the same time. They should be matched for manufacturer, capacity, and, as much is possible, manufacturing lot. When you put your batteries in a flashlight, it is critical that they all have the same state of charge (SOC). Don't mix half charged and fully charged batteries.
The reason for this is so that the batteries age together, hopefully remaining matched. When they are matched, they discharge at approximately the same rate. If they are not matched, or if one battery has a lower SOC than the others, it is possible that one battery will be fully discharged in your flashlight before the others. This can be dangerous because the depleted battery will sometimes get recharged from the other batteries while you are using your flashlight. This is a process called "reverse charging."
Dangerous levels of reverse charging are much more likely to occur when batteries are connected in series than when they are connected in parallel. That's good news for you, because the batteries in the Olight SR52UT are connected in parallel. When batteries are placed in parallel, and all start with the same voltage, then the parallel arrangement tends to keep the voltage matched as the batteries are depleted. This is not so when batteries are connected in series.
If you bought your three Olight 2600mAh batteries as a group, all from the same vendor on the same date, they would have started out as a matched set. If you have used them separately, however, or recharged some of them more often than the others, then they may no longer be matched. That's because battery characteristics vary depending how often they are recharged and how deeply they are discharged or charged in use.
Storage conditions also play a role in keeping batteries matched. Batteries in a matched set should be stored together, so that they experience the same ambient temperature. They should be recharged together, so that when they are stored, they are all stored at the same voltage (i.e., SOC). In general, you want to recharge a Li-ion battery as soon as it needs it. Storing a Li-ion battery in a discharged condition will shorten its life. Storing one in a deeply discharged condition can even be dangerous when you attempt later to recharge it. For long-term storage, discharge your batteries to around 3.6 volts. Storing them fully charged or fully discharged will shorten their lives.
Some chargers, called "analyzers," have the ability to check the capacity and internal resistance of a Li-ion battery. This allows you to periodically test the batteries in a matched set to verify that they remain matched.
Best practice is to buy a new set of matched batteries to go into your new multi-cell flashlight. Protected batteries based on the Sanyo/Panasonic NCR18650GA 3500mAh cell are a good choice for the SR52UT. You can get them from KeepPower, Orbtronic, EVVA, Olight (?), and others. In the USA, Mountain Electronics sells the protected EVVA GA at a good price.
