Yes, you can crochet multiple times into one stitch. (Actually, you can knit/purl into a stitch more than once, too.) A lot of the most common crochet patterns, like the eternal granny square, depend on multiple stitches in one chain, space, stitch or what have you. So yes, you do make three single crochets in one chain stitch, then make a single crochet in each of the next four chain stitches (I'm assuming you're making the second size, as you said), half double crochet in one chain stitch, double crochet in each of the next two chain stitches, and put seven double crochet into the last chain stitch. That looks like the bootie sole to me, so donb't be surprised if the pattern tells you to work down the other side of the starting chain. By putting a bunch of stitches in one spot, you make the work round out. The scs are probably the heel and the dcs are most likely the toe. If you just do it, you'll see the pattern come out.