I think I ripped my hamstring or something by doing splits and it's still a bit sore after 3 weeks..

It takes a few weeks.

First you warm up by doing quick easy movements such as running in place or jumping jacks: just to raise the temperature of the muscles. Then you sit on the floor with the soles of your feet together. Take hold of your toes with both hands and gently lean forward as far as you can, pulling your toes toward your forehead. Hold for a second or two, then lean one inch farther. Hold for a few seconds, then relax. Now do it again, but this time go one half inch farther. Do it a third time, going just one quarter inch farther. Now stand up, walk around, and rest a little while. You can do this exercise as often as you like, but DO NOT try to hurry it. It takes three weeks to give noticeable results, and if you try to hurry it you will hurt yourself.