Sutra:
10. avyApakankaLil vyApakankaL mUnRum srEshtankaL.

Meaning:
The vyApaka manthras are considered higher than the avyapaka manthras, and three vyApaka manthras, ashtAkshari, dvAdasAkshari and shadakshari that contain the Narayana, Vasudeva and Vishnu namas, are considered high amongst these vyApaka manthras.

Sri PBA Swami's Sarartha Deepikai:
In the previous sutra, the manthras describing the Lord were seperated into two groups. Of these two groups, the vyApaka manthras are considered higher than the avyApaka manthras. Even amongst the vyApaka manthras, three manthras are considered as very important. The Vishnu Gayathri manthra is well known: nArAyanAya vidmahe vAsudevAya dImahi, tanno vishNu: pracodayAt". In this manthra, three namas of the Lord are said: Narayana, Vasudeva, Vishnu. These three speak of the all- pervasiveness of the Lord and as such are vyApaka manthras. These three manthras are special amongst vyApaka manthras and are higher than the avyApaka manthras.

These three are namas; so, how can we call them as manthras? They are namas, but they are the inner meanings of their associated manthras. Also by adding the words Om, nama: and the chaturthi vibhakti (Aya) to them, they become manthras. Thus these three namas in the Vishnu Gayathri manthra are equivalent to three vyApaka manthras.