Bare minimum? Is a one-light setup.

Personally, if this is your first time doing flash as an ambient-only photographer, I would NOT start with a 3-light off-camera studio setup. That's going from kindergarten to graduate school in one leap.

Here's how I tend to advise folks learn flash:

Step 0) [standing] Master M and the exposure triangle (hopefully, you already have this down). The reason I say you want to do this is because flash exposure is really different from ambient-only exposure. Your camera's meter can only read light that's in the scene. Flash isn't in the scene. So, just getting your meter's needle to "0" does not guarantee a good exposure any more. And unlike ambient, where this a specific given combo of the three exposure settings that will get you a good exposure, with flash it's a lot more complex.

You're essentially combining two exposures like two layers in Photoshop: the ambient exposure from all the light that's in the scene that isn't from the flash and the flash exposure. And those two exposures have different controls.

Ambient exposure, as you know is controlled by iso, aperture, and shutter speed.

Flash exposure, however is controlled by iso, aperture, power, and distance.

So you have two settings that affect both (iso, aperture).

One that only controls ambient if at or below sync speed (shutter speed).

And two that only control flash (power, distance).

And. Within gear limits in certain situations, you can balance the flash against the ambient however you want: everything from a silhouette in front of a well-exposed background to a well-exposed subject in front of a black background. And it's all going to be "well-exposed."

It's kind of like going from juggling three balls to juggling five while riding a unicycle. Just saying. And that's just intensity control. You also need to consider direction, quality, and color. Oh, and directing/connecting with your subject. :D

Step 1) [walking] Master on-camera bounce flash.

Put away your other two TT685s. Just put one TT685-F on your X-T4's hotshoe, and learn on-camera bounce flash. It's better than you think for portraits, particularly with a BFT flag to block any direct hard light.

Use Neil van Niekerk's Tangents website as a starting point. It's much MUCH easier to master the basics of flash exposure, flash/ambient balance, and controlling the intensity, direction, quality, and color [with gels] of the light with simple bounce flash, because all you have to master is the flash and which way to point the head. And all the additional gear you'll need is a $1 sheet of black craft foam and a rubber band. Granted, you won't have the same freedom with direction and quality you would have with off-camera flash, but you'll still have some.

You'll also have all the toys (TTL, HSS) to play with so you can make a judgement call as to whether or not you need them, and you'll be set for event/social shooting as well as chasing kids/pets around the house, where dragging a light on a stand behind you is not practical. Also, you can travel light with a flash. Off-camera generally means packing a lighting bag to go with the camera bag. Wait until you're sick and tired of there not being a bounce surface where you need one. Then, go off-camera.

Step 2) [running] Master a single off-camera flash and a single modifier.

Going multiple light from the very get-go is going to be too confusing and too expensive (remember, you also have to multiply stands, modifiers, etc. by the number of lights). You have to learn how to setup your stand, swivel/bracket, light and modifier; debug your triggering; direct a subject; and watch all that flash exposure stuff at once. It's a lot of load to tax your brain with, particularly when you're new to it.

You'll need:

• A lightstand. Like tripods, judge by how much weight you need it to support and how tall you want it go. Bigger/heavier/more expensive tend to be sturdier.
• A transmitter. I'd suggest an XPro-F if you're outside the US, a Flashpoint R2 Pro II-F if you're inside the US and can order from Adorama, or an X2T-F if you have to have a hotshoe on the top of your trigger. I would not look at older models, like the X1T-F where the UI and feature set are not as good. But you can also use a TT350-F or TT685-F as your on-camera transmitter. Their UIs, though are not as good as the dedicated transmitters for this job.
• Either an umbrella swivel/adapter* and a convertible umbrella (a white one with a removable black cover than can be used either as reflective or shoot-thru) OR a Godox S2 bracket and a Bowens-mount softbox are probably the most typical first-modifier choice. Umbrellas are cheaper and easier to setup/breakdown; but give you less spill control. If you do get a softbox make sure it mounts to the face of a monolight. You want to avoid the kind with a slit in the bottom for the stand to go through: those severely limit your ability to tilt the light.

Start with one light and one modifier, and get to the point where you can previsualize what that combo is going to do before you do it. Again it shouldn't take long, but it can also help you get enough experience to figure out what type of modifier you want to move to next, and also whether speedlights are sufficient for what you want to do, or if you need to move to bigger ministrobes or monolights. Your other two TT685-Fs might be just what you need. They may be not nearly enough.

*A note on swivel/adapters. I like the compact style of swivel because 1) you can just leave them on stands in the bag and 2) they often come with a screw-on spigot which can turn anything with a 1/4"x20 thread on it into something that can hold your speedlight (such as using a monopod as a light boom or creating DIY superclamps with Home Depot A-clamps and some bolts). But. They are much more of a PITA to adjust the tilt on than a full-sized swivel with a big handle. As for brackets, the Godox S2 bracket is better than the Godox S bracket because it folds up smaller, and can accommodate a Godox round head and has stepless tilt. YMMV.

Step 3) [flying] Okay, now go for broke with multiple lights/modifiers.

Hopefully by now you've got enough of a sense of what type/size of lights you'll want so you can get additional stands/modifiers accordingly [i.e., a monolight tends to need a more robust stand than a speedlight], and you can start juggling ratios for key/fill/rim/background.