Well, on Miro’s free plan, you can indeed create and keep 3 editable boards. However, if those boards have become inaccessible, it could be due to because you may have been added to another team, and your 3 free boards are tied to a different team space or Miro sometimes changes free plan rules, and boards might become view-only if limits are reached. But as far I think that it might be a temporary glitch or account sync issue could also lock access.

Do some effort, check whether you’re in the correct team/workspace (look in the left sidebar). If your boards are there but locked, it’s likely due to plan restrictions.

​@Darya Yershova The rule is 3 editable boards for the entire team and not each user. Can you confirm that, when you filter your dashboard to show all boards, there are other boards owned by other team members?

Perhaps you have created boards in someone else's team. As ​@Angelina Harriet suggested, you can find any other teams your Miro account profile is a member by using the team selector:

https://help.miro.com/hc/en-us/articles/360017571294-What-is-on-your-dashboard#h_01F8Q78MPKXM4ZJ0EJDD6C8Q0X

If there is another team space with no boards, you could move your boards there. 

There are additional steps you could take, but I'll wait to hear back after you've confirmed the details above.