I had this same problem and I think almost everyone does. The back of the hand is naturally weak. Keep your left hand ‘pronated’ well around and keep the base of the pinkie as close to the fingerboard as you can. Sometimes when third finger is played, fourth finger curls up and pulls down, almost going under the neck of the violin. Don’t allow this. Every play of the third finger, the pinkie should snuggle right next to third finger. This should be relatively easy to do. A good exercise is just to trill from 3 to 4, holding 3 down. When this is easy on A and E, try it on D and G. Elbow under! Next do it again from 2 to 4, holding 2. Same procedure. Then, 1 to 4. then 0 to 4. Determine never to let the finger collapse no matter what you play. I made this decision my first year in college. In about 2 weeks, I didn’t have to think about it much anymore.