BRIEF-Mexico economy minister says NAFTA negotiators face big challenges in next round

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Sept 27 (Reuters) -

* Mexican economy minister says third round of NAFTA negotiations was "highly productive"

* Mexican economy minister says encountered differences in some issues in NAFTA talks, will have substantial challenges in next round

* Canadian Foreign Minister Freeland says negotiations completed on chapter on small and medium-sized enterprises

* Canadian Foreign Minister Freeland says meaningful advances made in telecoms, customs and trade facilitation

* Canadian Foreign Minister Freeland says negotiators working from consolidated texts in most areas

* Canadian Foreign Minister Freeland, in reference to NAFTA talks, says "we never said this was going to be easy"

* NAFTA negotiators says next round of talks will be in Washington from oct 11-15

* NAFTA negotiators say expect to conclude the negotiation on competition chapter before the next round Source text for Eikon: Further company coverage: 1/8 3/8 (Reporting by David Ljunggren)