Easy ways to check yarn tension:

Once you have stretched the yarns for the 3 strands, you can check the tension

1. • by visually checking that no yarn is longer than the others: the longer one(s) sag and you can easily see them as they’ll stick out
   

2. • by gently applying the palm of your hand on all yarns, and determine whether some yarns feel tauter than other
   

3. • for the musically inclined: lightly pluck each yarn and see if you get the same pitch - that works better if your setup can act as a soundbox as it will amplify the sound.