Some labs produce in concentrations of 10 mg/mL, else it's indeed pretty much impossible to dose that low. Unless, as Logan mentions, you dilute it further yourself, which I can easily see going wrong by contaminating it in a non-negligible number of cases.
Having said that, 3 mg weekly has little to do with gaining muscle if that's your goal as a woman. While it will put your testosterone a bit above what's normally seen in healthy, menstruating, women, it's of little help for gaining muscle.
As known, testosterone is very potent for muscle-building given in the right dosages. Give hypogonadal men TRT and you'll see increases in LBM and strength. Give eugonadal men gradually increasing dosages of testosterone in the supraphysiological range, and you'll see gradually greater increases in LBM and strength.
However, if you give women with low testosterone, as seen in menopause, 3 mg of testosterone weekly (which puts them above normal levels), you don't see any of this yet. Quite in fact, you don't start to see any changes in strength or LBM unless you go beyond 12.5 mg weekly:
To determine dose-dependent effects of testosterone on sexual function, body composition, muscle performance, and physical function in hysterectomized women with and without oophorectomy. 71 menopausal women who previously underwent hysterectomy ...
www.ncbi.nlm.nih.gov
The threshold is just relatively higher.