Your post is so vague, and you dont even explain the scenario you are complaining about in detail.
Either way, if a adc goes in and kill 2 assassins, in a straight 1vs2...the adc always dies unless the assassins are bad skillwise or the adc is completely fed and the assassin's are too behind and even so I dont think the adc can survive.
Now if you are saying in a real teamfight situation there are too many factors that you need to pay attention:
1- The Adc support; If its a Lulu, Soraka; Janna, etc...The truth is, it becomes very hard to kill the adc without a good initiation by your team;
2- Team comps; If they got a Galio, Ivern, Gragas, even a Elise or Lee that can peel for a adc, your assassins obviously will die doing no damage. You need to plan the teamfights and get picks, most importantly be smart. Split them, Catch one that was warding or wandering, etc...
3- Fed and feeding; If the enemy adc is fed and your assassin's are under leveled obviously they wont do much especially when the adc has someone that can peel for them.
4- Skillwise; I think this one is self explanatory;
5- How the game is going and how the communication of your team works; Objectives are important and there are ways to ambush people or make them come to you. If the bot lane has a big wave coming to their tier 2 tower, obviously they will have to send someone to kill that wave. Most of the time its the adc that goes to clear the wave, so this is a chance where you can kill him. Another one is bait Baron or Drake if you got the vision set up.
Conclusion: There are various ways to play around in League. I suggest you actually take time to improve your game knowledge instead of complaining in the forums where you wont get much out of it.