I would go for option 2. If I had all three (unfortunately only got OT so far), either one would be the beat/rhythm unit in any/every case. Depending of how you want a song/jam to start, the others would then compliment each others very good in that regard.

Want to start with acoustic/sample based sound (beat and/or melodic): OT.
Want to start with analog sound (beat and/or melodic): A4.
Want to start with digital sound (beat and/or melodic): MD.

Since all got extensive parameters aviable, many other types of sounds are possible to acheive in the others regardless of which one you start with.

Even if they all got plenty of knobs, you don´t need to tweak all of them all the time. I think it´s about being fluent and openminded to what type of sound you want to bring up next in your jam session and being experienced enough to get it from any of your unit.

Why not try to have sessions for yourself where you switching the “roles” each unit got?

One will be the (background) beat, second will be the (background) melodic and the third will be “your” instrument that you´re playing/tweaking more than the others. Then in the next session, shift their abovementioned roles.

I think all of above can be had more or less if going for option 2. That option doesn´t make the MD & A4 redundant because of that you CAN sample them with your OT. Option 2, IMHO, gives you the choice of how you want to use them together no matter of how you want to start your song/jam. That means alot of flexibility.

My 2 cents…