Instead of 3 strands of 4 or 5 weight yarn, I used 2 strands of 5-weight chunky yarn (Deborah Norville Serenity Chunky, colors were blue and sailboat, I think). After some fits and starts with the starting chain and first few rows -- my first attempts were far too wide, so I kept restarting trying to make it smaller and more manageable -- this blanket worked up really quickly. It's so thick and plush -- the perfect spot for baby's tummy time. The corners curled up on me, and I'm not sure if that's just due the pattern or a result of my inexpert crocheting, but it's not a huge design flaw that I feel I need to troubleshoot if I ever make this blanket again.
I mixed two colors of my favorite yarn for baby items, Bernat Softee Baby. As with the other blanket, I had to restart this one a few times until I was happy with the size, but the good thing about crochet is that it works up quickly, so ripping out and restarting doesn't feel as futile as when you're knitting. With this yarn, I like to make the blankets less than 30 inches square. It's a manageable size to tote around, and it works well for tucking around baby in the car seat or swing.
Here's Oliver at 2 months, as happy as can be.