If you trimmed off 3", then you need the new squares to be 3 1/4". They'll be a lot more stable if you leave them as squares and follow the directions in my first post. You added this while I was typing. Still try it with paper first. Starching that bias seam is a good idea - press by lifting your iron - don't push it at all.

Cool I want to try the snowball quilt too. Thanks for sharing!

For the smaller squares ... do not cut them into triangles til after you have sewn them on.
Plus oversize them, so when they flip out and are pressed, you will have excess to trim off and can be sure the snowballed block is nice and square.

The challenge you will have ... the corners cut off the large block, were they cut to precision or not? If not, when you assemble all the snowball blocks the points will not line up.

You have some lovely fabrics ... I hope you can get it all to come together!
Good Luck!

I've found this way is best for me. Sometimes the smaller square I sew on is a little off but by not cutting the big squares corner off my blocks sew together without any problems.

I would start over and use the ones you have now for a different quilt when you get a little more experience.
Welcome from Missouri. These nice knowledgeable people will help you.

You could "rescue" your quilt by cutting four 2 1/2 " squares for each of your blocks using a complimentary neutral color. Mark a diagonal line on the wrong side of each each of these squares. Place your small squares right sides together with your larger square just as though you had not cut the corner off. Stitch, fold over and press. I would not remove or trim the bottom little triangle, but would leave both layers for stability. You might have to still trim a bit to get all blocks a uniform size, but not much. Hope this makes sense. Don't feel bad, we were all new quilters at one time. Personally speaking, I have "salvaged" more than one project. It may not be what you had originally planned, but will be lovely all the same. I like your fabric. Good luck!

Correction to my previous post as I just read that you cut 3 " corners. Adjust the size of you small blocks to 3 or 3 !/4 to make it work.

One thing I find is when you are sewing if you will sew just a little to the side of the line towards the corner ( just one stitch length), when I press it it just seems to be the right size, not too short. I learned this is a quilting class and it has always worked for me.

......... and what is known as the scant 1/4" :D

In the old days, snowballs were made by cutting the corners off first, then sewing on the triangle

My 80 yr old aunt had a fit when she saw how I was making a set once, "you're wasting so much fabric! Your grandmother would be rolling in her grave if she saw this." She didn't like any of the "new fangled" way of cutting out quilts. Cardboard templates or templates made from milk jugs with sandpaper glued to the back, a pencil and a good sharp pair of scissors was all you needed. I wonder what she'd think of these new die cutters.

In this case I would:
1) cut down the main square to fit the triangles I have and then add borders to make the project big enough or

2) cut new small squares big enough then sew the too small triangles together to make some sort of border for this project or some other project or throw them out or

3) bag up the whole project and start over, congratulations you have your first UFO.