Crochet "in side" of stitch

I have just begun to crochet the outside frame of a dresser scarf whose center is a series of assembled medallions. The first round of the frame consists of chains with the occasional tr or dtr attaching it to the medallion pieces. The corners of the first round are composed of (dtr, ch7, dtr) and the first round ended with a tr in the fifth ch of beginning ch-19 to form the last loop.

My question is about the instruction for round 2 of the frame. Having just completed the first round as above, it says: Ch3, 3 dc in side of same st. What does "in side" mean -- through a front or back loop? I am assuming it doesn't mean "3 dc in same st" because I did a lot of shells in the same stitch while forming the medallions, and they didn't use the wording "in side." Any help will be appreciated.