Yes, it is possible to have guests during the concept discovery on the morning of day 3 of a design sprint. This will depend on the team’s preferences, but it’s not without risks:
These newcomers may sometimes have a high hierarchical level (sponsors) and can impact the decision-making process or even create some apprehension or concern among team members.
If they are ‘fly on the wall’ visitors, they may still slow down the process as it will be necessary to explain the first two days of the sprint in detail to them.
With these warnings in mind, it can sometimes be beneficial to invite additional experts on day 3, such as technical experts who can quickly assess the feasibility of the concepts.