Knowledge of spelling development concerning phonology, orthography, and morphology and the non-linguistic factors affecting spelling execution have significant instructional and theoretical implications. Few studies have been conducted on Binisayang-Sinugboanon regarding spelling and literacy development and primary pupils simultaneously learning Filipino and English languages, which vary in the phonological, orthographical, and morphological systems. This study was conducted to provide an in-depth understanding of the triple word-form theory and cross-linguistic interference by explicitly analyzing the linguistic features and spelling error categories in the expository paragraphs in the Binisayang-Sinugboanon, Filipino, and English of the multilingual fourth-grade spellers. Types of spelling errors were identified and interpreted using the POMAS Coding System and Analysis of Variance (ANOVA), in which the Binisayang-Sinugboanon got more errors in orthographical and morphological features among the three codes, which implies that multilingual participants find their mother tongue very complex in terms of their spelling structures. All three codes were high on orthographically-based errors with many occurrences in capitalization and unusual vowel patterns in English and vowel errors for both Binisayang-Sinugboanon and Filipino. Loanword fusion, a newly discovered error feature, was high in both local codes- proving the effect of multilingualism on spelling. The results of this research have transparent instructional implications for teaching spelling and theoretical implications for multilingualism concerning literacy development. The study may pave the way to research-based instructional intervention, emphasizing the orthographical-phonological-morphological interrelatedness, which may vary pedagogically considering their effects on multilingual learners.Knowledge of spelling development concerning phonology, orthography, and morphology and the non-linguistic factors affecting spelling execution have significant instructional and theoretical implications. Few studies have been conducted on Binisayang-Sinugboanon regarding spelling and literacy development and primary pupils simultaneously learning Filipino and English languages, which vary in the phonological, orthographical, and morphological systems. This study was conducted to provide an in-depth understanding of the triple word-form theory and cross-linguistic interference by explicitly analyzing the linguistic features and spelling error categories in the expository paragraphs in the Binisayang-Sinugboanon, Filipino, and English of the multilingual fourth-grade spellers. Types of spelling errors were identified and interpreted using the POMAS Coding System and Analysis of Variance (ANOVA), in which the Binisayang-Sinugboanon got more errors in orthographical and morphological features among the three codes, which implies that multilingual participants find their mother tongue very complex in terms of their spelling structures. All three codes were high on orthographically-based errors with many occurrences in capitalization and unusual vowel patterns in English and vowel errors for both Binisayang-Sinugboanon and Filipino. Loanword fusion, a newly discovered error feature, was high in both local codes- proving the effect of multilingualism on spelling. The results of this research have transparent instructional implications for teaching spelling and theoretical implications for multilingualism concerning literacy development. The study may pave the way to research-based instructional intervention, emphasizing the orthographical-phonological-morphological interrelatedness, which may vary pedagogically considering their effects on multilingual learners.