Do you think that a 3.0 will damage my chances at a 1L summer job and job down the road given how much emphasis is placed on the 1L grades?
Well, that question is subject to a number of variables. To a large extent it depends on what your goals are. If you have a 3.0 from Harvard, then no worries. If you have a 3.0 from Whittier, however, and are trying to get a prestigious and highly competitive federal internship, then it may be more difficult. Without knowing what your goals are it's hard to say.
As far as post-law school employment, again it depends. Big firms with 500+ lawyers and some federal agencies are going to be far more competitive than the local public defender's office (although those are probably more competitive than you think!). If the goal is a small firm doing family law and DUIs, it will be less of an issue.
Also while I have you here, I can't afford to do an unpaid full time internship, so if I don't get a paid internship or a part-time unpaid internship, would it be devastating for me to just work for a company I worked for in undergrad?
I don't know what the market is like where you live, but I can tell you that here in California paid internships are very competitive. The majority of students get unpaid internships. Sadly, firms and government agencies have figured out that there is a huge pool of talent willing to work for free.
I worked at a government office when I was in law school and we only budgeted for a small number of our internships to be paid. Obviously, the competition for those positions was especially high.
If you have to work at a non-legal job during the summer, so be it. However, it is imperative that you get some solid, marketable legal experience before you start looking for a job as a lawyer. The job market is very tight, and employers will be flooded with applicants who have experience. Those without experience will be at a serious disadvantage. Again, the caveat might be if you are graduating from a school with a huge reputation and you can simply rely on pedigree (Harvard, Yale, etc).
Otherwise you better have something else that the employer wants, namely experience.