Can I call this a 3.5-way system?

Do you plan to apply a shelving filter like this?

Screenshot from 2022-11-13 16-18-24.png
 
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No, use one 3-way speaker per side and run both full range and below 100Hz at the same time.
I'm easily confused also.
A point five woofer is [ as far as I know ] always passive and with an extra woofer per box. If you are Bi-Amping then it surely must be a 4-Way.
You can't run full audio spectrum [ 20 to 20k ] and sub100hz at the same time in the same box but you can run your 3-way 100 to 20k with separate woofers running from 20 to 100hz easily when Bi-Amping.
That's easy and all it takes is the extra amp and two more boxes with drivers but you can put the extra woofer inside the big box rather than separate. Most of us don't because big boxes can be rather heavy and unwieldy when moving furniture
 
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Adding? I'm not sure what you mean (*). I'm sure it could be done but I'm concerned it might cause some damage if not done the right way. Maybe you could draw a diagram...

I suspect you mean equalising, in which case it would not make a 3.5 way.

* Filters are usually strung in series and sent to (for example) a single output...

Screenshot from 2022-11-13 17-28-32.png
 
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OK I think I've got it now. You want to boost the sub100 Hz signal in the speakers.
It's just a 3-Way then.

But, I saw some speakers e.g. B&W Matrix 3 have one tweeter and two woofers where the two woofers have different crossover frequencies, if I recall correctly the upper and lower woofer are crossed at 3,000 and 400 Hz, respectively, and they called them 2.5 ways.

In my case, the three-way speakers are fed bass signal below 100 Hz filtered by an active crossover to them. Why not call them 3.5 ways?
 
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Adding? I'm not sure what you mean (*). I'm sure it could be done but I'm concerned it might cause some damage if not done the right way. Maybe you could draw a diagram...

I suspect you mean equalising, in which case it would not make a 3.5 way.

* Filters are usually strung in series and sent to (for example) a single output...

View attachment 1108985

It seems to be like the right image.

Bass below 100 Hz aren’t boosted, but are filtered by an active crossover.
 
My image is an active crossover.

Do you use one amplifier to power your 3-way speaker or two? Is bass below 100Hz lower or higher? Does it roll off or shelf? Is it sent to the same driver or different?

Two amplifiers are used because it’s bi-amp setup. Bass below 100 Hz means lower than 100 Hz. It should roll off as it goes through the low pass filter. And, it’s sent to woofers of the three-way speakers through the low-frequency amp.

In brief, the three-way speakers play full range, and the bass lower than 100Hz will be sent to them when the “constant bass” switch is engaged.
 
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