To determine if a sample is representative, we need to compare the proportion of its characteristics to the proportion of those characteristics in the overall population.

The population consists of circles (•) and triangles (▲).

• Number of circles: 6
• Number of triangles: 6
• Total population size: 12
• Proportion of circles: $\frac{6}{12} = 0.5$ (50%)
• Proportion of triangles: $\frac{6}{12} = 0.5$ (50%) A representative sample should ideally reflect this 1:1 ratio or 50% for each shape.

• Number of circles: 3
• Number of triangles: 3
• Total sample size: 6
• Proportion of circles: $\frac{3}{6} = 0.5$ (50%)
• Proportion of triangles: $\frac{3}{6} = 0.5$ (50%) The proportions in Sample 1 match the population proportions. Representative? $\boxed{\text{Y}}$

• Number of circles: 4
• Number of triangles: 2
• Total sample size: 6
• Proportion of circles: $\frac{4}{6} = \frac{2}{3} \approx 0.67$ (67%)
• Proportion of triangles: $\frac{2}{6} = \frac{1}{3} \approx 0.33$ (33%) The proportions in Sample 2 (67% circles, 33% triangles) do not match the population proportions (50% circles, 50% triangles). Representative? $\boxed{\text{N}}$

• Number of circles: 3
• Number of triangles: 3
• Total sample size: 6
• Proportion of circles: $\frac{3}{6} = 0.5$ (50%)
• Proportion of triangles: $\frac{3}{6} = 0.5$ (50%) The proportions in Sample 3 match the population proportions. Representative? $\boxed{\text{Y}}$