Li-ion battery Charging Issue
3.7 V is purely the nominal voltage. The working voltage is typically around 3.2 to 4.2 V. 2.8 V is lower than you would like but not throw the cell away low.

They should be trickle charged until the voltage recovers back to the normal range but after that they should be fine. Which a decent charger should do automatically.

Obviously all the the normal caveats about begin careful with unprotected Li-ion cells and charging / discharging them using the correct limits and protections still apply.