This is a great topic! Thanks @missy and @lyxzen
Ok so I think there is WAY more than three things you should know how to do if you wish to successfully adult, but if I'm really breaking it down and can only choose three things... they are;
1. Be alone. - so many people can't sit and have lunch alone, or go to the movies by themselves. It's considered taboo, or people think you don't have any friends. So not true! Solitude is a must! Once you've mastered the art of being alone, I feel you can truly appreciate when you spend time with someone. I feel that it also helps you to realise your self worth.
Everyone finds peace in solitude in different ways, whether it's reading a book or just sitting on a park bench enjoying the sunshine. In this there is no right and wrong, as long as it works for you (and doesn't break any major laws haha).
Which brings me to.....
2. Love yourself - Yes, a hard one. But really, if we can't learn to love our flaws and our quirks then we aren't living our life to its fullest. That big booty? Love it! The not 'perfect' teeth? Own them! There will always be days of self doubt and self loathing, but if you learn to love yourself then these days become few and far between. We must remember that NOONE is perfect, and normal is just a setting on the dishwasher.
That thing you don't love about yourself is probably something someone finds endearing. Maybe you could ask a friend their favourite thing or feature of yours, or ask your lover what they most love about you. Looking at yourself how others see you can be hard, but learn to listen to the people building you up, and ignore the critics.
3. Work on their own car - As a car girl I see too many men and women that don't know how to change a tyre or service their own car. This should be a requirement of getting your licence. Now I don't expect you to be able to fully rebuild your engine, or change your gearbox... But knowing how to provide the upkeep on your own car, and how to change a flat, will help save you money and get you out of trouble.