I'm not sure if this is possible, but, on the 5D3, taking a photo with mirror lockup is a 2 step affair - pressing the shutter button flips the mirror up, pressing the button a second time takes the picture.
Is it possible to lock the mirror up once and take subsequent shots after that? I'm thinking of my old mechanical film cameras (F-1, EF) where the mirror lockup button would do just that.
My reason for asking is that using the TC-80n3 timer (say, taking a shot every 30 seconds) kicks up the mirror, but not the picture. Maybe I'm doing something wrong here?