Back to the main section of this seed, from the spawn just diging straight down and after a while you will reach to the a entrance of large cave which leading you to the cave system.
After reach to the cave system you have two options. One of them is closest to the mineshaft. This mineshaft doesn’t safe because it contain lots of lava and fire, so careful for each foot step if you don’t want die soon. The first diamond very easy to finding just put the fire and lava to see it.
From the location you finding the first diamonds, start mining the stone blocks arounding. And you can find two more diamonds. Next steps, turn back to the other path and start searching from there. Don’t worry about you can’t find the diamond because it hiden inside the stone because most of them you can see without mining.