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Let $G=(X\cup Y\cup Z,E)$ be a 3-partite graph such that:

  1. $|X|=|Y|=|Z|=q$.
  2. $2 \leq d(v) \leq 6$ for all $v \in X\cup Y\cup Z$, where $d(v)$ is the degree of v.
  3. $\sum_{x \in X} d(x)=\sum_{y \in Y}d(y)=\sum_{z \in Z} d(z)$

Can we partition $G$ into $q$ vertex-disjoint triangles?

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A recent paper by Custic, Klinz, Woeginger "Geometric versions of the three-dimensional assignment problem under general norms", Discrete Optimization 18: 38-55 (2015) contains (and proves) the following proposition:

Proposition 5.1: Partition into triangles on 6-regular tripartite graphs is NP-complete.

Now just follow the reduction in the proof with a NO-instance, and you will get a 6-regular tripartite graph on $3q$ vertices that cannot be partitioned into $q$ triangles.

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