Parenting teenagers is a doddle if you've worked in a zoo or on peace keeping missions in Iraq. Most of us haven't done that, I have learned the hard way, if any of this is at all useful, knock yourself out.
Don't watch Skins or Hollyoaks, especially Skins, it will fret you.
Don't ask questions that you won't like the answer to. I work on a strictly need to know basis. I'm sure that's much better for my peace of mind.
Find a good hiding place for your stuff. Teenage girls in particular think that any cosmetics/perfumes/toiletries that are in the house belong to anyone that needs them. You will buy everything and use nothing. This goes for shoes, clothes and jewellery too, it is much worse if you are the same size, your 17 year old daughter looks so much better in all your clothes. Teenage boys think all food in the house belongs to them, no matter how many times you tell them that if it's a fruit and nut toblerone it's yours, they'll still eat it.
Music has to be played at 1 decibel below ear bleeding to be fully appreciated by the teenage ear. Train them early to like your music, it hurts less when you like what's being played.
Choose your battles carefully - sure where's the harm if they look like they got dressed in the dark or have been at the dressing up box. Take pictures, lots of them - in years to come you'll have hours of enjoyment looking at the cut of them.
If you put food in the fridge, it will be eaten. If there is beer, it will be drunk. If your house gets the rep of having such items you will be descended on by a plague of hungry/thirsty teenagers. This is expensive.
They appear not to get tidier with age. You would think that having 3 teens you could delegate some of the chores out, spread the weight a bit. You can try. Good luck with that.
The behaviour level is sometimes similar to the toddler phase, especially when the hormone fairy has visited, they are significantly less cute though.
What excited them when they were 5, will excite them again at 16/17/18. They love trampolines, bouncy castles and jelly. If you do a birthday party with kids games like pass the parcel and provide alcohol, you will have some very happy teenagers.
Remember when your adorable little cherub of a toddler got you up at 6am every day ? It is incredibly enjoyable to go into a teenagers bedroom (double the fun if they're hungover/woke you up when they stumbled in drunk/vomited in a bathroom cleaned that very day) and wake them up. You can bounce on their bed, open the curtains and say in a nice cheerful tone "S'morning now, morning now, get up, get up, get up, I'm wake now, entertain me, hungry now, bored now, can we go to the park ?" or any other little saying your cherub was fond of in their pre dawn years.
Love from your