No actually.
Consider:
Fist as a PRIMARY weapon means that you are not triple wielding - in this case, you're absolutely correct.
BUT, in the case that an MA is using Kuma Tonfa (I have to assume that's what we are talking about), then here is how it works (generally):
three attack bars:
2 for kuma's
1 for combined MA attack
Now, you have a valid question: isn't the combined MA still the primary weapon?
Here is the answer for you:
In continuous fighting, you should - if your inits are good enough, be 1/1 with all weapons, which means a total of 45 attacks/minute.
Broken down, that's 30 attacks for kuma tonfa, and 15 for combined MA
Kuma tonfa's, fully buffed and at MBS are hitting for about 5k, which can go up to about 6k with add damage.
Combined MA fully buffed with endgame armour will hit for about 9k or 10k with a bit of extra add dmg
This means that kuma tonfas are on PAR with combined MA (for comparative purposes kuma dmg = 5000*30 hits = 150000/min and combined MA is 9500*15 hits = 142500/min) - close enough, roughly that they are equal.
But, the difference comes from fast attack. With 2x kuma tonfa equipped, it's a safe assumption that the MA doesn't have Bow/piercing IP'd, in which case they'd have more likely spent the IP in fast attack.
At capped fast attack, you get an extra 10 hits per minute with the kuma tonfas:
5000*10=50000 dmg/min
Now, while the tonfa and MA from proportional numbers of attacks per minute were reasonably close, with the fast attack included, the contribution from kuma tonfas is significantly more:
200000 dmg/min (from tonfa) vs 142500 dmg/min (from combined MA)
*Note: numbers are rough estimates only, but should serve to illustrate the difference. I've assumed critical rate is 100%.