Mandarin tone 3 sandhi refers to the phenomenon whereby a tone 3 syllable changes to a tone 2 when followed by another tone 3. This phonological process creates a deviation between the tonal forms realized at morphemic (/tone3-tone3/) and word ([tone2-tone3]) levels, posing questions in terms of how disyllabic tone 3 sandhi words are represented and accessed. The current study conducted three cross-modal lexical decision priming experiments to investigate this issue. Experiment 1 manipulated the frequencies of the initial morpheme and whole word, showing that the higher initial-character frequency against the whole word gives stronger activation to the underlying representation and the lower frequency of the initial character leads to stronger activation of the surface tone. Experiments 2 and 3 operationalized the relative frequency of the initial tone 3 morpheme's realization as a sandhi tone, finding that the competition between the two tonal realizations also influences how T3 sandhi words are accessed. Specifically, the more frequently the T3 morpheme surfaces as a T2 allomorph, the less activated the underlying representation becomes in the mental lexicon. Our results indicate a complex interplay between morpheme, word, and the associated tonal representations in the mental lexicon and that these factors co-determine the lexical access of tone 3 sandhi.