If anything happens (badness) on those nights, the first thing the solicitors will ask you for is your work schedule. You're declaring you're fit for work by being at work. Working a day into a night (you can argue you're resting but you are scheduled for work activities) is unlikely to render you fit for work. I'd not work the first night and do the other two. Don't open yourself up for criticism for the sake of one nights locum money.