You should expect some 3 unit questions require some methods or formulas from 2 unit as a part of the solution, which the steps that is before or after the 2 unit method are beyond 2 unit course. An example of this is that the equation sin^2(x) + cos^2(x) = 1 is also commonly used in the trigonometry proof of 3 unit course. It is also possible that part (i) is a 2 unit question, then part (ii) is a 3 unit question that requires part (i) to solve. There may also be a few questions that can be solved purely using 2 unit method, however, it is either easier by using 3 unit method, or relatively difficult for most 2 unit students and requires more abstract thinking.
In term of the percentage of 2 unit content in 3 unit paper, I'm not sure and I believe it varies from paper to paper