Well this new mechanic don't look very bad. But, if I was towering right now and attacking this tower, then i'll tell my members just to "rush flag". For the people who are in a towering guild, you'll hear this "rush flag" a lot in the guild chat while attacking a tower. It means ignore all the defenders outside the flag room including any mechanics/strategies they're using. I believe sacrificing three attackers is too much unless if it's not a war guild between +18 members in each guild. It also wouldn't be wise to decrease the number from 3 to 2. To be honest, I wouldn't even bother standing there, I would just head straight to the flag room with my members ( If I was still towering)
It's not a bad idea to Have players standing in a certain position to activate something. However, I don't think it's a good idea to have players standing on something to shortern the way to flag room to their guild members, it's just not worth it.
Therefore, i'll list some suggestions that could help:
1-having at least six attackers standing in this position to create the HP cell in the defenders spawn. Then, you won't have to take the entire tower's space to fill it with boxes like the current one. Which will obviously give you the chance to fill the tower with other attacking/defending mechanics/strategies.
2- After certain defenders stand in this position, it'll activate something like fire fences that won't allow the attackers to reach the flag room because everytime they'll attempt to pass through the fire, they'll obviously get hit. Basically, it's similar to the switches and blocks but, harder to activate; killable.
3- After certain players stands in that position, lizzardons will be summoned into the flag room!!

I can't think of any other suggestions right now, i'll edit the post later to add other suggestions.

I was only joking about the third idea.