His acknowledgment that the Divine Being is 'single' is only verbal
But let us resume the examination of his words in order. “Each Being has in fact and conception a nature unmixed, single, and absolutely one, as estimated by its dignity.” Why “as estimated by its dignity?” If he contemplates the Beings in their common dignity, this addition is unnecessary and superfluous, and dwells upon that which is obvious: although a word so out of place might be pardoned, if it was any feeling of reverence which prompted him not to reject it. But here the mischief really is not owing to a mistake about a phrase (that might be easily set right): but it is connected with his evil designs.
He says that each of the three beings is 'single, as estimated by its dignity,' in order that, on the strength of his previous definitions of the first, second, and third Being, the idea of their simplicity also may be marred. Having affirmed that the being of the Father alone is 'Supreme' and 'Proper,' and having refused both these titles to that of the Son and of the Spirit, in accordance with this, when he comes to speak of them all as 'simple,' he thinks it his duty to associate with them the idea of simplicity in proportion only to their essential worth, so that the Supreme alone is to be conceived of as at the height and perfection of simplicity, while the second, in proportion to its declension from supremacy, receives also a diminished measure of simplicity, and in the case of the third Being also, there is as much variation from the perfect simplicity, as the amount of worth is lessened in the extremes: whence it results that the Father's being is conceived as of pure simplicity, that of the Son as not so flawless in simplicity, but with a mixture of the composite, that of the Holy Spirit as still increasing in the composite, while the amount of simplicity is gradually lessened. Just as imperfect goodness must be owned to share in some measure in the reverse disposition, so imperfect simplicity cannot escape being considered composite.
Source: Against Eunomius (New Advent)