Practice hands separate.
I agree with the other above poster. Don't practice FFF for loudness. Practice FFF for strength. Strength of sound comes from depth of texture. Textural depth is rooted in the integrity of inner and lower voices. Don't trick yourself into voicing the top note of the third louder in an effort to make the chord sound louder. This will lead to playing brightly when you should be playing broadly. Instead, practice by exaggerating the strength of inner voices.
In your particular case, it sounds to me that you may also have insufficient upper arm/forearm stamina to support the repeated actions you are attempting. The following is what I would suggest to build the necessary strength:
First, disentangle tension from strength through relaxation:
First, hold up your hand straight. Visualize your fourth finger as an extension of your forearm. Using a stroke originating from the spine or shoulder and proceeding down your entire arm, play a single note E on your fourth finger of your right hand at medium volume, at no volume in particular. No matter what that note ends up sounding like, attempt to relax your entire arm and shoulder on that side as completely as you can while holding the note to the bottom of the key. You should be able to lean forward from your bench, from your spine and abdomen, into the piano and feel every muscle system in your arm and shoulder acting as a series of springs. They should all compress together as you lean in, and extend together as you lean out, without any joint or muscle system taking what feels like a disproportionate share of the burden. Practice the single whole-arm stroke and relaxation until you can reliably and quickly reach relaxation in between strokes
Once you are comfortable with the single stroke and relaxation at no particular volume, then begin to control the volume. Continue to experiment to find the way in which you can play and hold a single note most loudly, most softly, while relaxing most completely and immediately after striking the key.
Next, separate loudness from strength through voicing:
Now add the C beneath it on your second finger. Strike the C-E third at no particular volume until you can relax properly. Pay particular attention to togetherness.
Once you are comfortable, strike C-E pianissimo. Over repeated strikes, while maintaining correct relaxation between strikes, increase the volume of the E to fortissimo, increase the volume of the C to fortissimo, decrease the volume of the E to pianissimo, decrease the volume of the C to pianissimo.
Your eventual goal is to find and become comfortable with the range of voicings from pp to ff that makes the C-E third sound like a third, instead of two voices. This step involves more training of the ear and mind and less of the strength of the playing apparatus. I find that when you play a gradient of repeated notes form pp to ff, the E should seem to crescendo first and less, the C second and more.
Once you have separated both tension and loudness from strength, then build strength:
Strike the C-E third twice, at your desired volume and in your desired voicing, relaxing in between the two strikes. Do not repeat the note faster than you can relax, or this drill will not work. Gradually diminish the interval between strikes, while maintaining relaxation between the strikes and the whole-arm attack on each strike. (The whole arm attack may be much diminished, but the action must still be able to transmit force from your spine/shoulder all the way to the tip of your finger.)
Add a third repetition, and a fourth. Through the addition of more repetitions and the shrinking of the interval between them, get up to about 9 strikes in a row at upwards of 6 strikes per second, maintaining full relaxation between strikes.
At this point you should be able to comfortably do relatively musical drills. You should endeavor to build a combination of muscle strength by intense, short sets, and muscle endurance by less intense, longer sets.