As with all exercises, play it slowly with a regular beat. Play it higher up the fingerboard to reduce unnecessary stretches.
As soon as any tension or strain is felt, stop and have a break.
Building an exercise like this in to your daily routine will have a huge impact on the playing of pieces. When you play a piece you won't need to spend time focusing on manipulating the 3rd finger into place as all that hard work has been done in the exercise previously, thus allowing you to focus on making music.