Lithium polymer battery numbering
Li-ion batteries usually follows the naming convention according to size, so for example an 18650 battery is 18 mm in diameter and 65 mm in length. 0 denotes cylindrical form.

In the past, a tailing 5 wound indicate a prismatic cell but yours doesn't.

Anyhow, my best educated guess is that 35 denotes the length, 08 the width and 3 the thickness and all measurements in millimeters.