Crochet 3 treble stitches together as follows –
~yo, hook in next st, yo and pull through, yo and pull through 2 loops (2 loops on hook),
~yo, hook in next st, yo and pull through, yo and pull through 2 loops (3 loops on hook),
~yo, hook in next st, yo and pull through, yo and pull through 2 loops (4 loops on hook),
~yo and pull through all loops.