When it comes to being in water, it does not matter whether one adds ammonia as NH3 or ammonium as NH4. Once it hits the water it will change based on the pH and Temperature of the water. (Salinity will also have an effect, but we are talking FW here.) What we are usually measuring with an ammonia test is Total Ammonia (TA) which is the sum of NH3 + NH4. When one adds enough ammonium chloride to a tank to produce 3 ppm of total ammonia, they are essentially adding 100% NH4. As soon as it can circulate some you can test and get a a reading of 4 ppm of TA. However, given the proper equipment or data on TA, pH and temperature, one will discover some amount of the TA is not longer NH4, it has turned to NH4.
The above works if the ammonia added top a tank is in the form of 100% NH3. Put in enough NH3 to create 3 ppm of TA into the same tank as above and in short order you will have the exact same balance betwwen NH3 and NH4 that resulted from adding all NH4. Here is an example. Two tanks, identical in every way possible except that Tank A has a pH of 8.0 and Tank B has a pH of 7.0. Each have 3 ppm of ammonia added to them. Lets also look at each tank using two different temperatures as well. 75F and 82F. Here is what you will discover;
Tank A: 3 ppm TA, pH 7, temp 75-> NH3 = 0.0159ppm, NH4 = 2.9841ppm. Raise the temp to 83F and NH3 = 0.0215ppm, NH4 = 2.9785ppm
Tank B: 3 ppm TA, pH 8, temp 75-> NH3 = 0.1513ppm, NH4 = 2.8487ppm. Raise the temp to 82F and NH3 = 0.0215ppm, NH4 = 2.9785ppm