Our firm is in trouble.

hi

this is all understandable

I've edited it below

good luck with the job

Our firm is in trouble.

The firm I work in is in trouble recently, our business is very bad, and can't even afford to pay salaries now.
Recently my boss is always losing his temper, but he was a good tempered person before. 
A lot of colleagues have already left, but I think I shouldn't.
The firm was created by my boss three years ago. I was the first member of staff after my boss. 
After I graduated, I had joined this firm, till now is three years. ...( structure of this sentence is awkward -  you could say ‘I joined this firm after I graduated three years ago’)
I have learned a lot from my boss and have grown a lot in these three years, so I can't leave, I'll stick it out through these hard times.
If I had known this three years ago, I would still have joined the company, because my boss deserves my decision.-(‘ deserves my decision’ is awkward, you perhaps could say  ‘deserves my loyalty’? It doesn’t quite mean the same thing though. I think you were trying to say something like ‘getting to know and work with my boss has confirmed that I made the right decision’
If I were to leave as soon as the  firm got in trouble, I would be a bad person.
I believe everything will be ok.

(your boss is lucky to have you!