The pink swatch is 2 strands of size 10 held together as one. The white is size 3. Both swatches were worked with the same size 0 steel hook.
I think they are close enough to say the two strands of size 10 make an acceptable substitution for size 3 (at least for this pattern, where gauge is not critical.) They are actually even closer in size than they appear in the picture. When I place one on top of the other they are almost identical; the pink is just a hair smaller.
But does anyone know where I can find the information on crochet thread size equivalencies for America, UK and Australia? I have sold several patterns to customers in other countries, and I want to be able to help them if needed.