Please Please Help! How do you do a dc3 tog?

Hi, I'm new to this site and am a self taught crocheter. I am trying to make a blanket. It wants me to DC3 tog over the next 3 chains, how do I do that? I have looked in my book-crochet for dummies and online on youtube trying to find out how to do that. I have tried it different ways and every time I get to the second row about 1/4 of the way through it starts puckering, not laying flat like in the picture. Here's exactly how this part of the pattern reads;
1st row-- 2 dc in 4th ch from hook, 1 dc in each of next 3 ch. (yoh. draw up a loop in next ch. yoh and draw through 2 loops on hook) 3 times. yoh and draw through all loops on hook-DC3tog made.
The next step- DC3tog over next 3 ch,1dc in each of next ch, *3 dc in each of next 2 ch, 1 dc in each of next 3 ch, (DC3tog over next 3 ch) twice, 1 dc in each of next 3 ch, Rep from * to last ch, 3 dc in last ch. Turn.

I'm just at such a loss as to how to do this, can anyone help me?

Thank You,