Koiné Greek & earlier lacked initial <μπ>, <ντ>, or <γκ> although these strings are commonplace word-internally. There are however a small number of Modern Greek words beginning <μπ>, <ντ>, or <γκ> that are inherited from Koiné Greek or earlier
Originally, the letters <β>, <δ>, & <γ> were used to represent the voiced stops [b], [d], & [g], but then around the time of Koiné Greek, these stops spirantised in most positions
About the same time, the originally voiceless stops <π>, <τ>, & <κ> voiced after a nasal, which was also one of the few environments where <β>, <δ>, & <γ> retained their status as stops. The nasal part of these nasal-stop clusters was also often dropped from speech, as the preservation of the voiced stop was sufficient to show the presence of the earlier nasal
But then in Byzantine Greek you start having an influx of loanwords beginning with voiced stops [b], [d], & [g]. To spell these, the scribes decided that the closest native segments were the word-medial [(m)b], [(n)d], & [(ŋ)g], and so used the same spelling for initial [b], [d], & [g] as they usually did for word-medial [(m)b], [(n)d], & [(ŋ)g]; that is <μπ>, <ντ>, or <γκ>
From this, we would expect that no words with initial <μπ>, <ντ>, or <γκ> are native, but this actually isn't quite the case!
There are a few words findable on wiktionary beginning with these digraphs that are in fact native. In each instance I can find, the Ancient Greek etymon has the proclitic ἐν- "in" (with assimilation of the nasal). It seems that in at least some cases, the vowel of this proclitic was lost, but with the nasal still causing a voiced stop
As an example, we have μπαίνω < ἐμβαίνω < ἐν-βαίνω, where the root verb is also preserved into Modern Greek as βαίνω with the expected fricative onset