You may have heard जुहोति ‘he offers (into the fire)'. The root is just हु — yet the word seems to contain हु twice. Class 3's trick: reduplication in the present itself.
When श्लु has applied, the root reduplicates. Abhyāsa rules follow: हलादिः शेषः (7.4.60), and कुहोश्चुः (7.4.62) turns the abhyāsa ह् into a palatal — झ् — which अभ्यासे चर्च (8.4.54) then makes ज्: हु हु → झु हु → जु हु.
The three third-person forms show the whole system: pit ति allows guṇa (जुहोति), ṅit-like तस् blocks it (जुहुतः), and the vowel-initial अति of the plural triggers यण् (जुह्वति).