Backing up: A monophthong (mono 'one', phthong 'sound') is a single pure vowel - for example, the vowel sound in standard English pronunciations of the word bit. A diphthong is, if you will, a moving vowel, as in standard English pronunciations of the word house. And yes, there can be triphthongs: the standard British and Australian pronunciations of the word fire would qualify, since the pronunciation passes through vowels like those in cot, bit, and but. I can't use phonetic transcription here, but each of those sounds has its own phonetic symbol. The terms monophthong and diphthong are strictly and solely about sounds - about pronunciations. So the word fear in British and Australian English (and in New England English) consists of a consonant /f/ plus a diphthong consisting of the vowel sounds in bit and but. In American English, however, fear is better analyzed as consisting of three phonemes: /f/, /I/, and /r/. The term digraph, in contrast, has to do with spelling - how sounds in English have come to be represented graphically. In the American English version of fear, <ea> is a digraph that represents the pure vowel /I/. In British and Australian English, in contrast, <ear> in fear is a trigraph (three letters) that represents a diphthong. All of which suggests that the term digraph is one that you as teacher need to know but that it may be less than totally transparent for your students. What about asking them to match the missing spelling, instead of the missing digraph?