In the '2' version, the third one is just on the inside of the case - no need to replace with a 3-on-the-outside version IMO... the version you have should work just fine.
In the 2 configuration one of the antennas that was originally placed inside the ST16 in the 3 configuration was moved to the exterior. That's all it is. Kits are available to do this on your own from Terrestrial Imaging. Or you can send them your ST16 and they'll do it for you. They even have a YouTube video that takes you through it.
There is no reason to extend the internal antenna to the outside on the standard set up. They have changed the antenna location on any “pro”versions with Real Sense and added some length to the whip ends. The ST reflects that “upgrade” by the 3 antenna versions you see. Range boosting upgrades need to have the 2nd flight 5gHz antenna brought out to the exterior so I’ve turned a couple of my “standard” STs into “pro” with these boosters kits. I bought extra whip antenna so if I want, I put on the normal screw on antenna or fiddle with mounting up the booster and attach with the coaxial cables. The booster antenna typically require upgrading the antennas at the H itself and extend those to the outside of the shell and screw on mini straight antenna whips. It’s funny when you get the booster kits as you get no instructions on installing or mounting the hardware.
What’s interesting is I’ve never lost flight control under the standard originally made 2 visible antenna first versions of the 480. The ones I’ve upgraded were for better camera viewing range and increasing the flight range just comes along with adding a booster set up. Carolina Dronz is currently running a Labor Day sale on all Itelite boosters and I think the 480 set up is $99.00 right now and that’s a good deal.