i lost alot of matches i play nocturne top because most of the time people play btm and mid.What should i do under these situation?

1)Mid And Bottom Keeps Feed And Enemy Vayne And Enemy Fiddle is 3 Level above me.
2)How do i cs at top or bottom if there is one champion with me also farming.
3)How to carry team?

1) Not much you can do, just farm I and try to roam. Try and let your teammates get the kill.
2) what?
3) You can't really carry as nocturne top.

This is all my Opinion, so it won't always help.

I mean it's really hard to answer these questions.
1.If mid and bot are both feeding to the point that they are 3 levels above you, there really isn't anything you can do. You could gank their lanes and help get a few kills.
2.If you are the ADC (Attack Damage Carry) you should be getting the cs bot. Generally top is a solo lane (most likely at your level it's a duo because there is no jungle) so it's quite hard to get cs when there are 2 people in lane
3. Nocturne isn't supposed to be a top in the first place (he is played as a jungler) so that already puts you at a disadvantage and top champions are MOSTLY bruisers that are not meant to carry but to take damage (eg Shen, Nasus, Malphite, etc...). This being said it is much harder to carry when playing top lane, instead the APC (Ability Power Carry) which is mid lane and the ADC which is bot lane should be carrying. Hope this helps.