You can import tasks from Basecamp 3 by clicking the "Import tasks" button on the top of your tasks or by right clicking on a task and selecting the "Import tasks" option.
Select then "Basecamp" in the list of import options and click "Connect to Basecamp".
You will be redirect to basecamp asking you to sign in(if you aren't already) and to give your permission for Planless to get your Basecamp data.
Once done, you will be asked to select your Basecamp's projects you want to import. Select the projects you want to import and continue.
If your Basecamp projects contain Team members assigned to tasks, it will be asked to you to match them to existing team members, create new ones or ignore them.
(Be aware that planless doesn't have multiple assignees per tasks. If you have a task with multiple assignees in Basecamp we will import the first one of the list as the task assignee).
Once done, just select the destination of your import and finish your import.