Select the alternative which represents three out of the five alternative…

Concept: A figure-formation item decomposes the target shape into exactly three regions with no gaps and no overlaps. Because each candidate piece may be rotated or flipped before being placed, the real test is not how a piece looks in isolation, but whether its exact edges and angles are congruent to one of the three decomposed regions -- including any concave notch on a piece, which typically becomes an internal seam against a neighbouring piece rather than part of the final exterior boundary. A piece that is not congruent to any of the three regions is a distractor, even if it superficially resembles the overall target shape.

Step-by-step application:

1. The diagram below shows the target triangle split into three regions: a tall right-triangle region running from the apex down to a point on the base, a second, wider region along the rest of the base that carries a concave notch on its inner edge, and a small filler triangle that plugs exactly into that notch (its two short edges become an internal seam once assembled, invisible from the outside).

2. Matching these three regions against the five given pieces by their exact edges and proportions: figure (1)'s flat right-triangle, figure (3)'s thin sliver, and figure (5)'s notched dart are each congruent to one of the three regions. Figure (2), although also a right triangle, has different proportions from the region it would have to fill, and figure (4) is an evenly-proportioned triangle that does not match the shape of any of the three markedly asymmetric regions.

3. Fitting figure (1), figure (3) and figure (5) together -- rotating each into its matching position -- exactly reproduces the three regions with figure (3) plugging figure (5)'s notch, so the exterior boundary comes out as a smooth, complete equilateral triangle with no gap or overhang.

Cross-check: Overlaying the three chosen pieces reproduces exactly the boundary of the target equilateral triangle with no leftover or missing area, while substituting figure (2) for figure (1) or figure (4) for either of the other two breaks that exact fit -- confirming that figures 1, 3 and 5 are the only valid combination.

The figure below shows the target triangle decomposed into the three regions filled by figures 1, 3 and 5: