I know the calculations can get complicated when adding fruit or lactose or whatever after fermentation is almost complete. It’s not too terrible, you just need to understand that it makes no difference when you add the sugar, you always need to treat it like it’s been there the whole time and add it to the OG. The other basic concept is of gravity units (GU), and for calculation purposes they need to be multiplied with the volume that they apply to, in order to keep everything on a parts sugar mass basis, then divide by the final volume at the end to get the final gravity of the final volume. You can always ignore the 1.0 in front of your gravity measurements and just deal with the digits after. So, 1.055 at 3 gallons becomes 55 * 3 = 165 parts sugar; 1.342 at 1 pint becomes 342 * 1/8 = 43 parts sugar, etc.
In your case, the extra pint of grape juice concentrate adds barely any volume at all (0.125 gallon out of 3.125 gallons total), however since it is concentrated, the sugar it adds (43 total parts sugar) is quite significant. As stated above, you get 165 parts sugar from the beer itself, then 43 parts sugar from the beer, for a total of 208 sugar points. Then this is divided evenly among the total volume of 3.125 gallons, so that’s 208 / 3.125 = 67 GU, or 1.067 effective original gravity.
So, your grape concentrate kicked up your effective OG from 1.055 to 1.067. Yes, you do need to find out your final gravity, because then, like normal, you’ll need to subtract OG from FG then multiply by 131 to get your alcohol by volume. If gravity heads down to about 0.999, then that’s (1.067 - 0.999) * 131 = 8.9% ABV
Hope this helps. It is a little complicated, but it makes more sense conceptually if you pretend the fruit concentrate was in the fermenter for the entire time. This mathematical method ignores time and pretends everything was always in there.