Brown would be an easy choice at #3 if some of  his weaknesses didn't look so troublesome.   Like crashing to the basket against 3 defenders, missing open jump shots, defensive lapses, just unfocused and wild at times.   He is explosive, quick but at times the game looked too quick for him. 

A good excuse he has is, Cal was dependent on him and other teams mainly prepared to stop him.  And because his team is mediocre (unable to buy a bucket in stretches), at times he had to try to go 1 on 3.  Opposing teams also could have played more zone D, preventing Brown to drive and have him shoot more jump shots. Defensively he was unfocused trying to do too much out there.

Now if he is surrounded by better players his positives could flourish much more and weaknesses masked better. 
Reading workout reports, his jump looks good, but I doubt he will ever be a very good jump shooter at the nba level.  With minor differences here and there,  I think Brown compares well to Derozan.  With unknowns - consistency and ability to come through in the clutch

The issues with drafting Murray is, we are very congested at the guard spot.  Drafting a #3 pick to only have him sit on the bench would be inefficient/unable to capitalise on the gift the Nets have given us. 

Murray should be automatic from the 3 pt line when open.   But it becomes more and more tricky to answer if he can get his shot off/able to score against good NBA defenders.  From JR Smith, to Tony Allen etc.
In my opinion he knows how to play the game. Like some have pointed about, he is like Paul Pierce in the way he uses headfakes, false steps, change of speeds and also has a very nice mid range game (floater, dribble stop and pops). 

Defensively regardless as to how hard he works at it, he won't be above average.  Like IT will never be above average. But both guys put in the effort at least and their IQ could bail them out here and there. 

One way to fit Murray in the lineup would be to trade Young and or Hunter.   Smart, Murray, Rozier off the bench backcourt (and  also if you want to go small ball route) could compliment AB and IT well. 

Comparison: Bradley Beal


Better player now: Murray
Higher upside: Brown

Maybe only thing Danny can do is flip a coin. Tough decision

Brown over Murray
Murray is better skilled now
But Browns potential is higher because of his athleticism and murrays lack of athleticism
Especially at the guard position where athleticism is greatly needed to defend and attack quick guards

Brown.

At #3 I Need upside. Murray at 3 is to high considering that he'll likely be a really good role player at best. I feel like it'd be using a number 3 on JJ Reddick. The NBA's rules cater to elite athletes and with Brown's tools at worst he's a rotational player. Not to mention, Murray's defense will probably keep him off the floor at least a season with Steven's defense first coaching style. At worst with Brown you get a Shumpert type, with Murray you might be getting a guy who can't guard well enough to stay on the floor. Then you have Brown winning the upside battle, so it's a no brainer. Not to mention the Celtics are almost completely void of any really athleticism.

But you know what could stink is ,

If we don't pick Murray and he ends up like Beal.  And Brown ends up like a Terrence Williams

Remember him? Super athlete but had no confidence in his jump shot, a little wild.  Just didn't have "it". You could tell he got by in College utilizing mainly his physical tools