ضَرَبَ has the root ḍ-r-b: three radicals, so it is triliteral.
بَعْثَرَ has b-ʿ-th-r: four radicals, so it is quadriliteral.
تَسَرْبَلَ contains five written consonants but does not have a five-radical root: the initial tāʾ is added, and its original radicals are s-r-b-l.
Counting visible letters is not enough. The morphological pattern identifies which consonants are radicals and which are formative additions.
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Three versus four root consonants
The Four Unaugmented and Augmented Quadriliteral Patterns
ضَرَبَ has the root ḍ-r-b: three radicals, so it is triliteral.
بَعْثَرَ has b-ʿ-th-r: four radicals, so it is quadriliteral.
تَسَرْبَلَ contains five written consonants but does not have a five-radical root: the initial tāʾ is added, and its original radicals are s-r-b-l.
Counting visible letters is not enough. The morphological pattern identifies which consonants are radicals and which are formative additions.