so will the 21B3 drive a pair of tri-wireable speakers better than a single 28B3?
I think depending on how much power the woofers and midrange section of your speaker required would be the determining factor. If your speaker has a very low impedance through the frequency range the 28B would be the better choice as it could deliver more current to that load.
If on the other hand your speaker is fairly benign through the frequency range and reasonably efficient then the 21B would be fine.
I am an advocate for more power to the speaker than less so I would choose one large amplifier driving a PASSIVE crossover rather than smaller amps tri-wired driving the passive crossover in a given speaker.
Now an Active system is a different matter altogether - an Active system like the Model T being driven by the 21B has huge performance benefits over the passive Model T being driven by the 28B.