How can I model a task that is performed by multiple roles?
Example 1: 3 roles have to meet in order to discuss an issue
Example 2: 3 roles collaborate in order to create a concept
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Your problem is thoroughly addressed in this excellent BPMN blog:
Activities cannot be placed common neither between pools nor lanes, and this is a common mistake. While this blog proposes possible solution I don't treat it as optimal and recommend you to check this answer on almost similar question as yours.
In my opinion you have to choose from these two options:
As it fairly stated in the above answer, lane semantics is not strictly described in BPMN standard, so we can freely address any entities through it.
You should choose the proper variant based on the semantics of your process.