I’m using Bernat Baby which calls for size 3 needle. I’m making a baby blanked but want to use 2 strands of yarn at once, so would I use a size 6 needle then? :??
It’s probably going to depend some on the pattern and some on the look you want. I’m doing a blanket in Baby Coordinates, calls for a 6, but I’m doing 2 strands together on 8. You could try swatching a bit in a couple different sizes of needles in the pattern to see what you like better. If it’s more of an “open” pattern you don’t want the needles to be so big to make it “holey” enough that a finger or toe could get caught. I don’t think it’s a “rule” to double the needle size when doubling up yarns.
The number after the ‘US’ has noting to do with actual size except that the needles get bigger as the number gets larger. A US3 is 3.25 mm and a US6 is 4 mm so it’s not really double in size. If you want to use two strands I’d start with a US10.5 (6.5mm) and then play around with different needle sizes from there. For a blanket, you probably don’t want to knit too tightly. Who knows, you could end up using a US9 (5.5mm).
Here’s a website with needle sizes: http://www.sizes.com/tools/knitting_needles.htm