Here in verse 15, the printer’s manuscript reads “while angels were ministering unto the disciples”, but the 1830 edition has the definite article the for the noun angels. In the previous verse, the angels are introduced; as a consequence, there is no definite article there: “and angels did come down out of heaven”. The most probable reading for the original manuscript in verse 15 is with the the. There is considerable evidence that Oliver Cowdery, the scribe here in 𝓟, tended to omit the definite article the (for a long list of examples, see under Alma 14:5). On the other hand, there is considerable evidence that the 1830 typesetter sometimes supplied an unnecessary the:
It turns out that the evidence from transmission errors is about equally divided. The critical text will therefore adopt the expected reading here in 3 Nephi 19:14—namely, the original text had the the before angels, which means that Oliver Cowdery accidentally omitted the definite article (either in 𝓞 or when he copied the text from 𝓞 into 𝓟).
Summary: Retain the 1830 reading in 3 Nephi 19:15 with its use of the definite article for angels (“while the angels were ministering unto the disciples”); the angels were introduced in the previous verse, so the definite article is expected in the subsequent reference to those angels in verse 15; there is considerable evidence that Oliver Cowdery could have accidentally omitted the definite article in this case.