Sounds like you have half square triangles. 3" on the 2 straight grain sides, and just over 4" on the bias edge. They should be 4 1/4" on the bias edge, though, unless the corners were trimmed off, in which case just over 4" sounds about right.
I actually prefer making half square triangle blocks by sewing on the diagonal of a square (two stitching lines 1/4" from a diagonal drawn line). There are also methods for making larger numbers of half-square triangle blocks.
But since they're already cut, I'd probably choose some kind of block that didn't just require you to sew two of these together, such as making some quarter square triangles to connect to the half square triangles, or something more complicated.