300 mg of test should keep you somewhat anabolic, but its also going to depend on how big of a calorie deficit you go on during your cut. drop calories too low and you'll still go catabolic with that and the T3.
you could try a Keto diet for the cut. Keto diets are 'anti-catabolic' by nature . when you get into 'ketosis' the body reverts to using only fat for energy , and will not tap into muscle proteins to produce glucose (as long as fat is in the diet and skeletal fat is readily available)