“How many mm of gum recession is normal?”

You didn't say how old you are, but age typically affects the level of gum attachment, as do other things such as bruxism (tooth grinding), occlusion (the arrangement of your teeth). 3mm of recession isn't terrible as long as it's stable and any factors influencing it are addressed.

Gum recession of any amount is not normal. Although common, gum recession is a pathological process that can lead to the loss of teeth. You should seek help from a local periodontist to determine which treatment is best indicated.

This is a little bit of a loaded question. Recession can be caused by a variety of things (movement during braces, a bad bite, clenching, and grinding, or other biological concerns). Depending on the source of the problem, the recession could be a real concern and can be addressed immediately. If the recession has been present for a long time and has not changed, this is usually a good sign and you may not require any intervention. If it is progressing in size or in symptoms, then it should be evaluated by your dentist to determine what action should be taken.