Consonant clusters combining a plosive and a liquid, rhotic or glide are relatively frequent. The only other type of cluster attested in an onset are combinations of the alveolar fricative plus plosive /st/, /sk/, but they are frequently reduced to just the plosive. Three consonants in an onset are not attested.
Source: Sandefur & Sandefur 1982: 61; 63
Source: Hudson 1985: 37