What can I do if my wife took off over 3 years ago and I can’t find her to get a divorce?

In a situation such as you have here, there is a legal remedy known as "divorce by publication". Before any legal action can be taken against someone, they must be given notice of the action so that they have an opportunity to appear in court and explain their side.  
A "divorce by publication" allows a spouse to be served notice of the divorce action via a newspaper notice. The filing spouse (i.e. the "petitioner) must make a dilegent effort to search for their missing spouse (i.e. the "respondent"). If they can present proof to the court that they made diligent efforts to uncover the location of the missing spouse, then the petitioner will be allowed to serve the repondent by publishing notice of the divorce in a newspaper instead of having to personally serving them (the judge will instruct the petitioner which newspaper to use; it will be in one that is in the area of the respondent's last known location). The respondent wiil then have 30-60 days to file an "answer". If they fail to do so, the petitioner can file a request that a "default divorce" be granted. As a general rule, it is granted upon the terms requested by the petitioner, although the respondent will be given a certain time period in which to appeal.
At this point you should consult directly with a divorce attorney in your area; they can best advise you of your rights and/or responsibilities under specific state law.