If you are experiencing clutch slipping specifically in 3rd gear under hard acceleration, it could indicate a worn or damaged clutch. While it's challenging to diagnose the exact issue without inspecting the motorcycle firsthand, here are a few possible causes and suggestions:
Worn clutch plates: Over time, clutch plates can wear down, losing their friction material and causing slipping. A thorough inspection of the clutch plates, both friction plates and steel plates, is necessary to determine their condition.
Weak or broken clutch springs: The clutch springs apply pressure to the clutch plates, allowing them to engage and disengage properly. If the springs are weak or broken, they may not exert sufficient pressure, leading to slipping. Inspect the springs for any signs of damage or weakness.
Incorrect clutch cable adjustment: Improper clutch cable adjustment can cause insufficient tension in the clutch mechanism, resulting in slipping. Ensure that the clutch cable is properly adjusted according to the manufacturer's specifications.
Oil contamination: Contaminated or incorrect type of oil in the engine can cause the clutch plates to slip. Check the oil level and condition, and make sure you are using the recommended oil for your motorcycle.
Given the symptoms you described, it is advisable to have your motorcycle inspected by a qualified mechanic. They can disassemble the clutch assembly, inspect the components, and determine the exact cause of the slipping. Based on their assessment, they can then perform the necessary repairs or replacements, such as replacing worn clutch plates, springs, or adjusting the cable as needed.