Giving my experience with this.
First, I unlocked MY BEST FRIEND upgrading the handgun and rolling back my saved game. Later when I thought I had enough gel, I tried the Crossbow first, because there isn't much to upgrade then went to the shotgun and fell short at the handgun again. Then I tried again, this time with the handgun and fell short. Then, finally having about 150k in gel, I tried again, and upgraded the handgun and POP goes the achievement.
So, in my experience, you have to upgrade 3 weapons, but it appears you do not have to have all three fully upgraded at the same time.
If somebody could confirm that would be great!
Hope this helps (and works!)