How to prepare 3 ppm solution of CdCl2

Check Cadmium chloride - Wikipedia, the free encyclopedia

This first gives you on how to prepare the salt.

Then, 3 ppm is 0.003 g/dm^3 (or g/L)

So, you need to put 3 grams of CdCl2 in 1 litre of water and you're done.