A 3-cell lipo pack measuring 2.5V is almost certainly beyond salvation. A new 3-cell pack should be about 11.1V when you receive it and should charge to 12.6V when fully charged.

Since the pack was delivered to you severely under voltage, the vendor should take it back without any hassle. Tell them you received the pack, checked the voltage, and it was only 2.5V. You should receive a replacement.

- Jeff