I think what you have done so far looks fine.
Now just pick one side and work it and leave the other side till later. It doesn't matter which side you pick, just work it so that the 3 stitches at the inside edge are knit (each row from here on out on the front edge is knit only--garter stitch) and the rest in rib pattern. If you are starting at the inside edge knit 3, then look at the ribbing pattern you have set up and figure out which stitch you need to do next to land on the correct stitch for the part you already have going. If you are starting at the outside edge, just rib across to the last 3 stitches and knit all 3 of them.
Your next instuctions say:
Working 3 sts at each center edge in garter st for front border and remaining sts in ribbing, work even on 21(23) sts of each side until armholes measures 3 ¼
That is what I tried to help you understand above. It may be telling you to work both sides at the same time, and you can if you'd like, but I probably would just do one and then the other. I get mixed up sometimes trying to work both at the same time, others love that method.
If you do them one at a time you will need to count your rows to try to have the number the same on each side, but if you are one off it won't be the end of the world. If you do them at the same time you need to be careful to keep the number of rows even too.