I have used James C Brett Pure Merino and a 4mm crochet hook.
The pattern is so easy you could probably do them in your sleep ...
Ch = chain
tr = treble
2 tr into 1st chain from hook, turn (3 sts)
Row 1: Ch 3, tr in each tr across, tr in top of turning ch, turn
Row 2: Ch 3, tr in 1st tr (inc made), tr in each tr across, 2 tr in top of turning ch (inc made), turn (5 sts)
Repeat Rows 1 and 2 until the triangle is as large as you require.