If you wait more than 3 years (i.e. you miss the statute of limitations deadline), then the Court can and almost certainly will dismiss your case "with prejudice." This means you can never file another lawsuit related to your accident.
You have 3 years to file a personal injury lawsuit in Washington. The time period in which you can file is called the "statute of limitations." This is a strict deadline.
Think of the statute of limitations like a clock. After your injury or accident, the clock starts. For example: if you are injured on July 1, 2016, you have to file your lawsuit no later than June 30, 2019.
Once you file your lawsuit, the clock stops. In legal terms, the filing of a lawsuit "tolls" the statute of limitations clock. At this point, you have an additional 90 days to serve the defendant notice of your lawsuit. This 90 day period is called the "tolling period." If you don't serve the defendant within 90 days, the tolling period ends and the statute of limitations clock re-starts.
Let's see this in practice: You're injured on July 1, 2016, which means the statute of limitations ends June 30, 2019. You file your lawsuit on May 30, 2019 (2 years and 11 months of time have elapsed). The clock stops, and the 90-day "tolling" period begins. You now have until August 30 to serve the defendant before the clock re-starts.
If you don't serve by August 30, the clock re-starts and now have 1 month (30 days) left to serve the defendant. Basically, once you file your lawsuit, you buy yourself an additional 90 days of "free time."
Let's say that you wait until the last possible day to file your lawsuit. Return to the example above. You file your lawsuit on June 30, 2019. You now have until September 30, 2019 (your 90 "free" tolling days) to serve the defendant. But if you don't, you have no more time left on the clock. The defendant will file a motion to dismiss your personal injury lawsuit for violating the statute of limitations time period, and the Court will dismiss your case "with prejudice." This means you can never ever re-file your case.