Ask HN: Tips for a prospective Ph.D student

I am a 25 year old guy who is going to be a PhD student in Europe. I have a bachelors in a pure engineering field (Ciivil), masters in operations Research and now my PhD is going to be in the field of Discrete Optimization. It is a three year program and I would like to solicit some advise from the community with respect to the following issues:

1) Any pitfalls that I have to avoid. My advisor is already decided and I can not change it.

2) It has been my life long dream to work for google and I think this PhD program is a good stepping stone. How can I utilize the next three years towards my preparation for a google interview?

3) Any other life advise?