You need to immediately hire a CPA who is competent in representing taxpayers who have tax problems, including unfiled returns. Preferably this CPA used to work for the IRS. As the other lawyers suggested, you should try to reason with the police department. However, on the assumption that the entreaties to the police prove unhelpful, you need to take action.
You will need to reconstruct your income and expenses from other sources, e.g., bank records; vendors; your memory. You want to construct tax returns for the past 3 years to the best of your ability and file them sooner rather than later. The IRS is unhappy with people who don't file returns. The IRS is extremely unhappy with people who don't pay their income taxes. You may have to file returns that include an attachment describing the tough situation with your files being held by the police. You may have to make a tax payment that is larger than you might otherwise owe were you in possession of all of the records. However, paying more than you might otherwise owe is a small price to pay for the fact that (i) you didn't file on time and (ii) you may not have kept copies of your files to guard against the loss of what you turned over to someone else.
What a miserable situtation. You might consider asking the police to make copies so you can make your filings (they may not return originals but perhaps copies)(Your request of course should be in writing.). You may consider advising now the IRS and FTB about your issue so your failure to file is not deemed intentional. Three years seems like a long time to release documents, and three years of non-filing could be a major problem with the tax autorities, as you already reallize.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
I agree with the prior comment, a request to copy the documents is reasonable. However, there is unreasonable bureaucracy. I would file an injunction and get a court to order access to make copies. You have a good chance of getting the order.
Information I provide in this forum is not legal advice. The information provided is only general information that may or may not apply to you and may or may not be based on current law. I am NOT YOUR attorney. For specific legal advice you need to contact an attorney and enter into a retainer with that attorney.