I have checked but could not find it. is there not a usual percentage for first, second and third year?
Usually the first year doesn't count to the final classification -- you just need to pass it. But I've seen a small number of cases where the first year counts 10%.
There really is no "usual" percentage weighting between second and final years. That said, I think I've seen the following splits in posts on TSR:
Some universities have multiple ways to calculate the final result. Sometimes there are special rules that apply to people near a classification boundary. Sometimes a university just looks at the best 100 credits in each year. Sometimes the mark is calculated in two ways and the student is awarded whichever calculations works out highest. There are probably other algorithms in use.