A single prism can produce a minimum deviation in a light beam. If...

To obtain the minimum possible deviation from a combination of three identical prisms, two of the prisms can be placed in opposite orientations (one inverted with respect to the other).

The angular deviation produced by the first prism is exactly canceled by the second inverted prism. These two prisms together effectively act as a transparent glass slab, which produces zero net angular deviation (only a slight lateral displacement).

The light beam then passes through the third prism, which will produce its own minimum deviation. Since all prisms are identical, the minimum deviation produced by this third prism is $\delta$.

Therefore, the minimum deviation that can be produced by the combination of the three identical prisms is $\delta$.