Being 3S and making for a "12V" system, they probably were overcharged. 3s @ 4.2 = 12.6. Most 12V chargers will push to about 13 or 13.5V. This would equate to about 4.33V or 4.5V per cell. Way over for Li-Ion's.
And a 4s would be too low and waste a lot of capacity. 4s @ 4.2 = 16.8, but most chargers only go to 13.5V, so you'd only have 3.375 per cell. If the charger goes to 15V, then you'd have 3.75V. Still undercharged. However, they wouldn't fail really. Just not have much capacity.