Personal preference is for sealed as I use electronic XO and separate subwoofers but properly designed ported boxes do give a lower response and +3dB
Yup. What is the length of the port in m? Double that, invert it (1/x) and multiply with 344 m/s. It will give you the port resonance frequency. E.g. for a port long 0.25m, the resonance is 688Hz.
The crossover from bass to mid has to be so low that the level of the bass speaker is at least 10-15dB down at this frequency. That will bring the port resonance down in level too. That, in turn, skips the need for elaborate damping of the bass enclosure just to get rid of all energy around the port resonance. And there you have one of the advantages of a 3-way system.