You are only guaranteed 4* and higher in the series packs. I have pulled step up packs in the past through and including step 7 and got a lot of 3* and 4* even at steps 6 and 7. If you only pulled one 3* at step 7 you are lucky. (I have incredibly bad luck, it seems, compared to others so I may not buy any more packs)
edit: It might be that at step 7 the only 3* are the reprint series (which would explain your experience) I can't remember that detail for sure. But I do remember pretty clearly that I got 3s all the way through, and that early in the steps I pulled a lot of regular 3* (and I pulled vast majority 4* throughout)