impact gun battery upgrade?

Hi,
i have a triton 12v impact driver with a battery pack of 3 Samsung INR18650-15L in them. I have 2 batteries for it, they usually do 1 week for me with light/moderate usage. One of them now only do 1-2days and i want to replace the 3 batteries in the pack, but reading here i realized they might be 'reviveable' or maybe only 1 battery is actually bad from the 3. How could i test which one is good or bad, with no special charger etc.? i have a multimeter and automotive battery tester.
Thanks

You need high current cells for that device. Replace all 3 with new Ones properly sized. Check what the c rating is for the current cells. Dont look at capacity because higher capacity is lower c rating and that equals more Tired tool

put a voltmeter on each cell to see what voltage it's at. If 1 cell is far lower than the others, that one(s) is/are dead. Don't bump start those cells. Especially in a powertool application. You could be creating a bomb at that point.

Once you find out what cell(s) are bad, replace them. Or just get new cells to drop in and replace all of them. This option I would recommend over replacing the individuals as the originals were matched. They have same life cycles. If you replace single cells, then they no longer are matched.

You can replace bad 15Ls with 15Ls or 15Qs.

If you want an upgrade, put some 25Rs instead (replace all) and you'll significantly increase capacity.
Samsungs 25Rs are in plenty of supply everywhere.

Thanks guys, i will go with the 25R-s and replace them all, it is still less than half the price of a new battery, happy days.