Yarn weight and Ply Question

I have a blanket pattern that calls for 3 mm needles and super fine (1) yarn. I bought the called for yarn but then realize that the pattern calls for 4 ply yarn and what I bought is 3 ply. Will this be a problem in the end project. Also guess I’m just nervous bc the yarn is super thin and have never knitted with such thin yarn before.

Ply is a very unreliable measure of yarn thickness. It refers to the number of strands that have been spun together but those can be of any thickness.
Make a swatch with your yarn and the 3mm needles as a starting point and see what your gauge is. Once you know what gauge you’re getting you can adjust the needle size to match or at least come close to the pattern gauge. For a blanket you won’t have to be spot on to be ok.

What pattern are you following?

I am actually not sure how to do a gauge lol I don’t have the pattern here with me but I will check and get back to u! Thanks for ur help!

If the pattern is listed on Ravelry, that page usually gives the gauge. Here’s a couple of videos on gauge that may help. It’s critical for fitted garments but not a bad thing to do for other items as well.

Definitely go by the gauge (which can be measured by swatching) comparing your gauge and the one on the pattern and yarn.