What is this plant that spontaneously sprouted in my balcony?
It looks like a raspberry, or some other kind of similar berry plant (blackberry etc.). The triple leaves and the habit of the plant are characteristic. Birds will often eat them and "deposit" the seeds randomly around the Earth. 

https://plantvillage.psu.edu/topics/blackberry/infos

Berry plants usually have thorns or develop them as they grow. If no thorns are present, then perhaps it is another type of interloping plant. Other plants do look similar. In which case, your photos are insufficient for more further identification.