You can use Translate and Scale Sphere[] as follows:
radii = Range[5];
Graphics3D[Translate[Scale[Sphere[], #], {#^2, 0, 0}] & /@ radii]
or translate to leave gaps between spheres:
translations = 2 Accumulate[radii];
Graphics3D[MapThread[Translate[Scale[Sphere[], #], {#2, 0, 0}] &, {radii, translations}]]
