You can turn your yarn into a triple weight strand as you knit, using the same general concept used in chain plying (or navajo plying) in spinning, looping the yarn through itself to make giant chains, essentially. It's super easy once you try it!
If you'd like video help, Alex Tinsley offers a video tutorial for this technique on her Dull Roar blog.

Use this technique on any leethal knitting projects when you want a yarn triple the weight of the yarn you want to use.

start by tying yarn end into a loop

bring yarn through the loop

then hold the loop and pull yarn through for a yard or so

so you have a length of triple stranded yarn, with the loop knot at the end

now grab single strand through the new loop at the end of the length of triple strand yarn

hold triple strand in one hand, and pull yarn through the loop with the other hand, to make a new length of triple stranded yarn; repeat this (pulling yarn through the loop at the bottom) every time you reach the new loop end as you knit

I like to pull and pull through each loop for quite a length (many yards), leaving a pile of yarn next to the ball, so that I can keep knitting the triple stranded yarn for many yards before pulling it through the next loop