Yes caira this is correct. Generally on day 3 or 4 is best. You want to catch your levels right after peak. If you draw on injection day your levels are close to baseline and this actually is the worst time to draw because it will reflect the lowest levels throughout the week. Generally it will take 24-48 hours to reach peak after you inject. See graph.
It is most important to get your blood draw on the same day (3 or 4) post injection every time you get a blood draw throughout treatment. This is called TRT restaging. This is the only way to truly pin point test levels during the week and find what your "optimal" dose is. It is done by analyzing the data under identical circumstances and comparing these data sets to each other over time