The difference between Cucumbers,Marrows and Courgettes

I thought I had some cucumbers growing in the green but am beginning to think they may be marrows or courgettes as I may have mixed up the labels.Is it possible to tell the difference between these three vegetables.

 

If you rub the leaves and try and detect the difference that way by feeling the smell.

 

If you have both cucumbers and marrows/courgettes, the cucumbers will have noticeably smaller first (seed) leaves. Cucumber seed leaves are normally smoother and slightly narrower than those of marrows/cougettes. When the first true leaves appear, those on cucumbers will be curlier.

And if any plants die at the faintest hint of a chilly night, they will have been cucumbers.

 

Marrows and courgettes, as far as I know, are basically the same plant, it's mainly about variety.
Cues are different and very easy to distinguish from marrows, as Scrungee points out.