I am ready to pull the trigger on a A3 Upper with Flashhider.  I know they are legal to a certain extent, but what conditions do I need to meet?

If you're doing a fixed mag version, the flash hider is OK.

If you're going the no-pistol grip version with detachable mag, then flash hider is a no-no (as would be a telescoping stock, forward pistol grip)

Do you know if flashhiders are removable?  or are they welded?  i am asking because i just bought a model 1 with flashhider and i was hoping they might allow detachable mags in the future.  if so i have to get rid of the flashider.

Quoted: Do you know if flashhiders are removable?  or are they welded?  i am asking because i just bought a model 1 with flashhider and i was hoping they might allow detachable mags in the future.  if so i have to get rid of the flashider.

If I'm reading that correctly when you mean "allow detachable mags in the future" I assume you mean the DOJ adding that receiver to the banned list, you registering it as an assault weapon, in which case you don't need to worry about it having a detachable magazine and FH. If it's a registered AW, it can have all the evil stuff you want (unless the legislature makes a new category of assault weapons)

Quoted: Do you know if flashhiders are removable?  or are they welded?  i am asking because i just bought a model 1 with flashhider and i was hoping they might allow detachable mags in the future.  if so i have to get rid of the flashider.

 It usually depends on where you got it from, and what length the barrel is. Most of the time, if you got a 16" or longer barrel, the company leaves the flash hider removable. But sometimes, when you order the 14.5" barrels that have a flash hider to make the 16" legal requirement, you will see them permanently attach the flash hider.  

 You could remove the flash hider, and everything will be OK. Just make sure to get something to protect the threads on the barrel.

Thanks for the great answers.  I wish everyone could answer as through and as fast as you guys.