There are two versions of the Roku 3, the 4200 and 4230. Your version was released in April 2015, so it's over eight years old. Both versions of the Roku 3 were outstanding players, and Roku still supports both of them with updates. That said, I wouldn't be surprised if they were the next ones to move to legacy status some time next year, simply because of their age. The 4200 is two years older than yours.
When you were on the system reset screen, pressing the OK button should have brought up a number pad to enter the numbers. But there is another way to perform a factory reset. On the back of your Roku there's a small hole that has a button recessed in it. Use a paper clip or some similar way to press that recessed button while the Roku is powered up and hold it for at minimum of 30 seconds. Not one second less, and ignore anything you see on the screen while you're counting. After the time passes, when you release the button the Roku will reboot with the new startup screen. It is now completely reset and you can safely give it away or recycle it. Just remember to remove it from your user account as well, as a precaution.