It let's you finish off the 3rd mission. But then the slot closes. Nice feature.
It's done this for at least the year Ive played. I know cause I dont use premium, only randomly receive it through events/etc.
Edit: cant type
Also for anyone like me. Pick the mission which you think will take the longest in the 3rd slot if you are on temp premium.
Note: Scratch the crossed out part, since if you finish one of the other missions. Still closes a slot. So doesnt really matter.
That's right, works as intended. Thank you for the right answer.