I am applying for a UK Standard Visitor visa. I have mentioned that I will stay in UK for three days.
What will be the minimum duration visa they will stamp, will it always be 6-month, multiple-entry or does it depend?
The Standard Visitor Visa is normally issued for a 6 month validity period, meaning you can stay for up to six months regardless of the length of the stay you applied for (if less than 6 months).
However, if you apply for three days and then stay for a month, you have just damaged your credibility for future visa applications, as the visa was issued on the basis of your short duration stay - this means that the evidence presented was looked at with the length of stay in mind, and a longer stay may have required firmer supporting evidence (showing you can support yourself in the UK for a week without working is entirely different to showing you can support yourself for a month without working).
Future visa applications will take into account prior visa's and visits under those visa's. If the application and the actual visit differ significantly, that will be noticed, regardless of the fact that your visa has a much greater validity period than your application was for.