In Jain dharm, for any karma‑bandh (including nikachit karma), the basic rule is:

• Karma binds to the soul through yoga (activity of

- mind – mano‑yog - speech – vachan‑yog - body – kaya‑yog ) plus

• kashāya (passions like anger, pride, deceit, greed, intense attachment/aversion).

So, about your exact doubt:

1. Do all three (mind, speech, body) have to act together?

No. For nikachit karma, just like for other karmas:

• Even one yoga is enough (only mind, or only speech, or only body)
• If that one is joined with strong kashāya, very intense and sustained, it can bind very strong karmas, including those that later become nikachit (tight, inescapable except by bearing their result).

Having all three yogas together and all filled with passions will generally mean heavier bondage, but it is not a compulsory condition for nikachit karma to bind.

2. What is special about nikachit karma then?

Nikachit (निकाचित) describes the state of bondage, not a separate “way” of binding:

• It is that portion of already‑bound karma which

- is very tightly fixed, and - cannot be prevented from giving its result – it must be endured (उदय सहना) only.

• How did it become so tight?

- Because at the time of its original bondage, there was intense yoga + strong, deep kashāya for a sufficient duration.

So:

• Cause‑side (binding time): any one of mind/speech/body (or a combination) with strong passions.
• Result‑side (when it matures): that part is called nikachit because now it cannot be changed or avoided, only borne.

You can read a clear explanation of nikachit karma and its tight bondage here

3. Digambar–Śvetāmbara view

• Both traditions accept:

- three yogas (mind, speech, body) as the channels for karma‑bandh, - kashāya as the main cause of strong bondage, - and nikachit as the tightest form of bondage which must be suffered.

• There is no major doctrinal difference here; only style of explanation and examples may differ.

Summary: Nikachit karma does not require simultaneous action of mind, speech, and body. It can be bound even by only one (mind or speech or body) when joined with very strong passions; that intensely bound karma later becomes nikachit, whose result cannot be avoided.