This is something that's commonly misunderstood. Public sector (defined benefit) schemes work differently to private sector (mostly defined contribution schemes).
Your 3% that you're getting from your employer is going directly into your pension 'pot' and invested in the stock market. Individuals in public sector schemes don't have a pension pot.
The police contribution of 31% is going into the overall running of the entire scheme. It has no bearing on what the retiree ends up getting because they receive a career average based on set rates when they retire.
For example, a civil servant contributes somewhere between 4.5-7.5% of their monthly salary. This buys you 2.32% of your salary every year and this accrues until you take your pension.
Let's say you earn £24k (£2k per month) and your contribution rate is 5% – that 5% (£100) buys you £46.40 (2.32% of your £2k salary) for your pension, or £464 over a year. If you work there for 20 years (assuming no pay rises/inflation for simplicity) you will have built up an annual pension of £9,280 (£464 x 20) which is paid out of the money built up by the employer contributions (27% in the civil service, and 31% in the police as per your example).
It is quite confusing as the media (Telegraph, I'm looking at you) conflate the public sector employer contribution and private sector contributions all the time - it certainly gets people riled up. But they are chalk and cheese.