Here is my opinion on how all three of your fish died
Yellow tang - to small of a tank
Clownfish - to many clownfish in a small space, two large clownfish turning female one killed the other to pair with the small one. When buying multiple clownfish one needs to be large and the other noticeable smaller, the only way you can have more than two in a tank is if all of the small ones are the same size and hope that none ever start turning.
Firefish - two firefish can't go together (except for the extremely lucky), the other firefish killed it. Even people who buy bonded pairs of firefish end up having one kill the other within a couple months, especially of they both want the same sleeping/hiding hole. Do a quick r2r search or google search about firefish pairs you'll see what I'm talking about.
Sorry for your losses, hopefully everybody else does ok. Just a heads up, when that damsel gets big it may get aggressive and start attacking and killing other fish. This will be about a year down the road and the entire tank will be its territory. You may want to consider trading it in for something else more peaceful.
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