×A 3t Gentryii is required at the bottom so that it does not slide off the pressure plate once you have everything else in place.

Consider the reverse. The only way to get a Gentryii on the top plate is to be in the region NW of it. If you then continue anticlockwise from there, the Gentryii will slide off, following you -- while you could theoretically use a Gentryii in front to stop this, there are 6 empty squares through which a Gentryii can slide to the SW, which is more than enough for the 2t and 3t to follow you through, no matter the movement order.

If you instead go clockwise to keep the top Gentryii in place, then the final Gentryii must be somewhere in the western part of the circle to lead it onto the plate (you fail if it's already in then southern part, because it'll then follow you up into the eastern part). However, as you reach the row containing the southern plate, the second Gentryii will slide all the way to (14,13), and you can't get west enough to stop that in time. So a 2t Gentryii there would slide off the plate.

The final possibility is that the 2nd Gentryii is in the NW corner of the circle (11,8 or 12,7), and so you must head west far enough to lead it out. But the 1st Gentryii will slide with it. If you actually try this, you will find that both 2t and 3t Gentryii will slide off the plate. It's a strategy that could've worked if the 3t Gentryii moved faster than the 2t Gentryii, but it doesn't. EDIT: Or at least, would've worked if you didn't have to worry about the center Gentryii too, but since it doesn't work in with the current movement order, you really don't have to care about that.

So you must find another setup, and the 3t Gentryii must necessarily be at the bottom plate.