What are these plants?

These plants (referred as A, B and C) appear in my garden. They grow very fast. Is that a problem?

 

Begonia
Peperomia pellucida
Possible Citrus

HTH
Chris

 

Little tree looks citrus to me as well. Lemon, orange.

 

Thanks to saltedar and luv2grdn!
The plants in the second and third picture must be Peperomia pellucida and Lemon as what saltedar said. However, I still have doubt with the first one, because it has a long red main stem which doesn't look like Begonia that I found in Google pictures.