I don’t understand what’s going on. I have a lof of cards that I press “Good” so it shows me that it will show, for example, in 3 months. But I see them again in two days. What is wrong with the app or with me?
Second is I press “Hard, 15 min” it can appear even in 3 minutes if I don’t have any other cards. Is that right behaviour?
I can only answer to your second question. Anki has a time tolerance (about 10 minutes by default if I recall correctly), meaning it will show you cards before hand if they were to be shown in the future, but by no more than that time tolerance, if you don’t have anything else to review. This is to avoid you finishing your deck, and then suddenly more due cards appear: instead, you’re better off doing all in a single batch. So, yes, this is the normal behavior, but you can change it if you don’t like it.
For the first question, when you see a card again that you don’t think you should have seen, please tap the gear/cog and choose Card Info. Do you see your recent review listed at the bottom? If not, you probably have Anki set up to create multiple cards with the same/similar content: Cards are reversed / appearing twice - Frequently Asked Questions