Is there in fact a neutral present? If so, then yes you may use it for a MWBC making 3 120V circuits out of the three-phase.

It's a wye with a neutral at the panel. I didn't intend to pull the neutral to the new 120V receptacles, just to land a hot on the neutral bus and color it white in the panel and boxes and use the other two hots to supply two 120V circuits. It's not residential work. My concern was that the neutral would never be balanced and that the 3P breaker would be used inappropriately.

You can't just remark the wire white. That's not permitted in this circumstance.

Are you, in fact, an electrician?

The situation isn't one of balance (even in the more common 120/240 single phase MWBC) but rather can the neutral have a current that exceeds its ampacity. You can have a completely unbalanced circuit where one leg has all the current and the other has none and Kirchoff's law shows that the 20A (let's say) leg can't flow more than 20A on the neutral. If there is more current on the opposite leg, the neutral has less.

It takes a bit more complex math with three phase, but the same thing happens. Imagine loading only one leg up to 20A, the current has to go back on the neutral. If you load up two legs to 20A, (and leave the third disconnected), no more than 86% of 20A will return on the neutral.

Thanks, Ron. I am, in fact, an electrician. I was worried about balance and appreciate your response. I had not thought of creating 3 120V circuits out of the 3 phase as you said in your earlier response. I found an abandoned 120V circuit that suits the purpose (a fan motor circuit for an unused freezer), so I won't use the 208V circuit, but you tell me why this circumstance doesn't permit reassigning an unused hot as a grounded conductor if it is marked white, 12 gauge, and moved to the neutral bus? Thanks in advance.

"but you tell me why this circumstance doesn't permit reassigning an unused hot as a grounded conductor if it is marked white, 12 gauge, and moved to the neutral bus?"

Because it is not 4 gauge or larger wire.

200.6 Means of Identifying Grounded Conductors

(A) Sizes 6 AWG or Smaller. An insulated grounded conductor of 6 AWG or smaller shall be identified by a continuous white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length. Wires that have their outer covering finished to show a white or gray color but have colored tracer threads in the braid identifying the source of manufacture shall be considered as meeting the provisions of this section. Insulated grounded conductors shall also be permitted to be identified as follows:

(1) The grounded conductor of a mineral-insulated, metalsheathed cable shall be identified at the time of installation by distinctive marking at its terminations.

(2) A single-conductor, sunlight-resistant, outdoor-rated cable used as a grounded conductor in photovoltaic power systems as permitted by 690.31 shall be identified at the time of installation by distinctive white marking at all terminations.

(3) Fixture wire shall comply with the requirements for grounded conductor identification as specified in 402.8. (4) For aerial cable, the identification shall be as above, or by means of a ridge located on the exterior of the cable so as to identify it.

(B) Sizes Larger Than 6 AWG. An insulated grounded conductor larger than 6 AWG shall be identified by one of the following means: (1) By a continuous white or gray outer finish.

(2) By three continuous white stripes along its entire length on other than green insulation.

(3) At the time of installation, by a distinctive white or gray marking at its terminations. This marking shall encircle the conductor or insulation.

http://freenec.com/T20.html

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310-15 (B)

(4) Neutral Conductor.

(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).

(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wyeconnected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).

http://freenec.com/T124.html

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210.4 Multiwire Branch Circuits .4 Multiwire Branch Circuits.

(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment.

FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.

http://freenec.com/T22.html

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HOWEVER, you should also read these;

http://www.nfphampden.com/mbc.pdf

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http://static.schneider-electric.us/docs/electrical%20distribution/low%20voltage%20transformers/harmonic%20mitigating/0104ed9501r896.pdf

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http://www.cromptonusa.com/Neutral%20Current.pdf

Thanks for pointing it out, Ron, and thanks for the citation, Fred.