The third hand is VERY visible. Doesn’t exactly have a lot of attention called to it, but it’s not like you have to squint and look in the exact spot for two frames, either.
Personally, I liked it. We’ve already seen that this T-1000 could do impressive stuff… why bother hitting the audience over the head with it? It’s a movie, not a grocery list of “Amazing Things Liquid Metal Killing Machines Can Do”.