The compositor for the 1911 LDS edition accidentally set come as came, probably because “came to pass” is much more frequent than “come to pass” in the Book of Mormon text (about 15 times more frequent in the early 20th-century Book of Mormon text). However, this passage is referring to the future, not the past, so come is clearly correct. The subsequent LDS edition (1920) restored the correct present-tense form.
Summary: Maintain the present-tense come in 3 Nephi 21:7 (“and when these things come to pass”).