Welcome to the forums cyph! Let's say you get a 1ML insulin syringe with 100 markings on it, and the T3 bottle is 30ml at 200mcg/ml. Each syringe unit would then be 2mcgs of T3.
If you needed to dose 30mcgs for a particular day, you'd fill the syringe up to 15 units. If you needed 50mcgs, you'd fill it up to 25 units.
Of course, there are the 10 bigger markers (10, 20, 30, etc.) on here too for when you have a nice rounded number. For example, if you needed 100mcgs, you could just fill the syringe up halfway to the fifth marker (50 units).