With time and even when new, many of the pins will will fall out and you really have to be on your toes. ...
The only exception to my above statement is the "Bolt Stop Pin" as I have only encountered one that had seen many cycles of hammering. I had one on a new reciever that was not welded in but stuck by the powder coating paint. ..
I take all 3 pins out, the trigger mech, and the bolt/spring.
No sir; there are only two pins, of equal size, holding the trigger housing/group, to the alum reciever. The bolt stop pin is the largest of the pins that you will encounter. The last time I looked, it is steel and fits through the Alum reciever. It has nothing to do with the trigger housing. ..
By the way, one thing I suspect, is that when bolt stop pin was removed, the bolt dropped down and you lost the position of the bolt spring and rod assembly.
Be Safe !!!