This is, indeed, formally a late view, and can be paralleled only by a few passages of a comparatively recent period.
Thus, in the late hymn i.164.

46: "Indra, Mitra, Varuna, Agni, they say; he is the sun (the bird in the sky); that which is but one they call variously," etc.

So x.114.5 and the late passage iii.38.

7, have reference to various forms of Agni.
Indra had a twofold nature in producing the union of lightning and Agni; and this made him mysteriously great.

But in Agni is found the first triality, which, philosophically, is interpreted as a trinity.
The fire of the altar is one with the lightning, and, again, one with the sun.