One (non-optimized) solution for a Stack Overflow question.

RemoveLastTriplet.cs

using System;
using System.Collections.Generic;

/*
Example 1
input : nums = [2,4,2,2,7,5,6,7,8,6,6,2,6,7,6]
output : nums = [2,4,5,6,8,6]

Example 2 input : nums = [2,2,3,2,3,2]
output : nums = [2,3,3]
*/
					
public class Program
{
    private static int[] RemoveLastTriplet(int[] input)
	{
		var toRemove = new bool[input.Length];
		var counts = new Dictionary<int, int>();
		
		foreach(var value in input)
		{
			int count;
			
			if (counts.TryGetValue(value, out count))
			{
				counts[value] = count + 1;
			}
			else
			{
				counts[value] = 1;
			}
		}
		
		foreach(var kvp in counts)
		{
			//Determine how many triplets we have for this value
            var tripletCount = kvp.Value / 3;
			
			//Keep track of where we're starting
			var currentIndex = input.Length - 1;
			
			//Remove each triplet
			for(var tripletIndex = 0; tripletIndex < tripletCount; tripletIndex++)
			{
				//counts the number of elements in this triplet
				var thisTripletCount = 0;
				
				//Mark each member of the triplet for deletion.
				for(var inputIndex = currentIndex; thisTripletCount < 3; inputIndex--)
				{
					if (input[inputIndex] == kvp.Key)
					{
						//Mark this index for removal
						toRemove[inputIndex] = true;
						thisTripletCount++;
					}

					//Keep track of where we are in the overall input array
					currentIndex--;
				}
			}
		}
		
		//We could be more clever here and keep track of how many
		// items we'll have in the output list and just create an array.
		var output = new List<int>();
		
		for(int index = 0; index < input.Length; index++)
		{
			if (!toRemove[index])
			{
				output.Add(input[index]);
			}
		}
		
		return output.ToArray();
	}
	
	public static void Main()
	{
		//var input = new int[] { 2,4,2,2,7,5,6,7,8,6,6,2,6,7,6 };
		var input = new int[] { 2,2,3,2,3,2 };
		
		var output = RemoveLastTriplet(input);
		
		output.Dump();
	}
}