About G Duel Infantry 3

It'll be 170, the description says "treats unit's stats as 170 in modes like Arena."

Merges don't affect you BST, a +0 Nino has the exact same BST as the +10 one.
In my opinion it isn't the best idea, Nino isn't a support unit and she doesn't have a prf weapon that needs a 300sp A slot to max her score

She'll be treated as a 170 BST unit (the same as +Spd Sheena or +Spd Gwendolyn).

Only do it if you use or like to use Nino on your Arena team, if you just rather not waste as many Crests for score sniping, or if you are on the border to Tier 20 -> 20.5 or Tier 20.5 -> Tier 21 and really want the extra rewards. Just realize that she won't be able to kill anything if you want to get the maximum score.

She would score as a 170 BST +10 unit.
Merges are counted separately from BST. Each merge scores the same as 10 extra points of BST even though it only actually gives 2 stat points.