Should You Add A Third Roller Post
NO, NO, NO.......If the idea is to improve the performance of the gate without altering the gate frame it self, you have just taken a giant step backwards by adding a third post with rollers.
Think about how the cantilever rollers work in relation to one another. When you see a gate moving into the closed position, if there is any gap at all between the gate frame and the cantilever rollers, you will see the weight of the gate resting on the bottom roller of the backframe post nearest the opening, while the gate will be pressing upward into the upper roller on the rear post.
When the gate rolls back into the full open position, weight shifts from the bottom roller on the front post to the upper roller on the front post and the rear post now receives the load on to the lower roller and no load on rear post upper roller.
Only TWO ROLLERS are working at any one time, now you are going to introduce a third post with rollers placed exactly between the front and rear posts.
Will you nullify the laws of physics? How many rollers will the gate ride on? The anwser must still be "two".
In effect, you have now reduced the effective backframe size by 50%. By adding the third post you have cut the backframe distance in half from you original design, because only two rollers are working at any one time, the front roller and now the new middle roller will pick up load.
The net result is your gate now is harder to move, NOT easier.