Students aren't required to pick all three choices

[keysmashes]

Also, I am not sure whether students have to select three projects. If that is the case, there is no warning when you log out if you only pick one, or when you just pick one as ‘second option’.

It's a valid usecase to select projects out-of-order (e.g. 2nd choice, then 3rd choice, then 1st choice), but potentially it's worth adding some UI element (maybe a sticky sidebar?) that shows just the titles of your current choices, maybe with missing choices highlighted somehow.

A warning on logout is unlikely to be helpful to many users, since usage of the logout button is presumably uncommon (though you never know – it would be interesting to get metrics for this), but a callout in the email receipt (see #9) that you haven't picked three choices yet might be a decent idea.

[keysmashes]

Alternatively, reminders for student_choice could be added, with the condition that they're only sent if the user has not yet picked three choices.

[keysmashes]

wsi-cogs/backend@81289cb includes a nudge in the email receipt that reminds students to select their remaining choices, if they haven't selected all three (see also 50340af). A sidebar (or some other UI) showing the currently-selected choices still seems like a good idea.