Presuming that the POCO can give you three phase, I'd say that you would have to replace the wire. Each of the 3 neutrals that in parallel will carry 1/3 third of the unbalanced portion of the total. This could introduce counter electromotive force CEMF or impedece in the triplex.
A good way to look at it in an extreme condition example. 100 amps on any one phase would put 50 amps on it's respective phase in only two of the triplexes and 33 amps on each of the three triplex neutrals. Another way to look at it is that 100 amps between two phase will have one of the triplex balanced while the other two triplexes will be severely unbalanced.
Each of the unbalance triplexes will generate a megnetic field around them and impede current flow. Worse case, something in the triplexes can burn up. The only way your proposed plan would work if there was an even load on all three phases all the time. That is why it is very important to have all the conductors of a circuit in the same raceway.
