Put in how ever much water you wish. Bac water is best, but you can use just sterile water as well.
If you use 2.5 ml, then 3 iu will be .25 ml.
If you use 1 ml, then 3 iu will be .1 ml.
If you use 5 ml, then 3 iu will be .5 ml.
If you use 3 ml, then 3 iu will be .3 ml
etc.