I bought eight of those very same "3 inch" casuelas from Hot Paella, via Amazon. They are nicely made, but I'm very sorry to report that the inside diameter is not 3 inches. They measure 2.5 inches inside diameter. The side walls are much thicker than what is shown in the picture of the 3 inch ones on their web site or on Amazon.
On the Hot Paella web site, they have both 2.5 inch (6 cm) and 3 inch (8 cm) casuelas. What I ordered and what appeared on my invoice was the 3 inch size. What they sent was the 2.5 inch size.
My St. Marcellin molds are 3 inch diameter at the base, so these 2.5 inch diameter casuelas are useless to me.
I suggest if you order them from Hot Paella, do it via telephone not via Amazon or via their website, and make sure they send you the right item. Mine are going right back to Amazon.