Where are you from? Would love to see a picture of your puppy!
Pixie is now 7 month old, so I still remember quite well the 3 months phase and all the lessons we went through in there.
You should start training her right away! Start with basic obidience: name recognition, looking you in the eyes, sit, lay, stay (I started there and find those good solid bases). Then you need to see what is a priority linked to your life style, for example for me is "go to bed" and have her staying there while we are at the table eating. The sooner you start working with her, the earlier she will learn (and you as well!).
As for the bitting, I tried EVERYTHING with Pixie! In the end what worked the best was to leave the room as soon as she bited me while saying to myself "how a bad puppy she was to bite me and that I did not like it..." with an upset tone. I like this article from Dr Ian Dunbar, take a look Teaching Bite Inhibition | Dog Star Daily
She still bits here and there now, but does not hurt me anymore since she knows my limit (I taught it to her). As for other things that she is not suppose to bite, that is more tricky... I always say leave it (now she does! Even the sleepears!) and give her something else to bite on. With socks I never tried the leave it, but it might be just too tempting for her to respond, she is crazy about socks too!
As for the walks 45 min might be a bot too much for such a small puppy (depends on your pace). The rul of thumb is 5 minutes per month (specially in cities with the concrete floor). When they are little they are a bit more bold and just go for it (reason why you should expose it to everything you can think of), then they start realiwing that there is a wild world out there and become more causions. Eventually they will build in confidence and it is fine again. Pixie had a period when she could not wait to go back in again, now she sits at the door with a face that says "is the walk over already?!".
For the peeing, are you taking her outside often enough? Also, very important, how are you cleaning the place whe she peed?
Puppies go thourgh different stages, we just need to get the best out of them.
If you have more questions and you think I can help, let me know...