A cat only needs one rudder,sounds to me like your servo or pushrods are not doing their job properly ?,if anything at speed a boat needs much less rudder input to turn than when running slow.Its possible with the central rudder you are putting a lot of strain on your servo and it may not have the power to do the job,most people don't use rudders directly behind the prop on surface drive boats especially when using large props,they cause too much prop wash for the rudder to work well .Adding the two outer rudders isnt the solution here,you need a high torque servo and good strong pushrods if sticking with the original rudder in centre. .
My advice would be try a stronger servo on the central rudder and remove the others and if this doesn't work then go with one single offset rudder on the right like most use,then you wont have any problems turning either way and your boat will gain some speed also as the rudder behind the prop will slow you down more than you realise.
Also you have too much up angle on your prop that isn't helping either,set it so it is level so all the thrust is directed where it needs to go rather than up in the air.It also looks like its quite high up ? when the boats sat on a flat surface with the rudders hanging over the end set the strut so the bottom of the strut hub is level with it then raise it maybe 1/8" and you should find it runs better there.