Here the 1830 edition has the modal verb shall in the first if-clause, but the printer’s manuscript has should. The shall could have come from the following clause (“that they shall believe these things”) or perhaps from the earlier use of shall in “when they shall have received this”—or more generally from the numerous instances of the present-tense shall (and one will ) throughout the passage. On the other hand, the should could have come from the immediately preceding clause (“that they should have first to try their faith”).
The Book of Mormon very often uses should in future statements. In fact, under 3 Nephi 1:8 I list four clear cases where the 1830 typesetter replaced the future should with shall. And to that list the critical text will add 3 Nephi 1:8 (as well as the case here of 3 Nephi 26:9). On the other hand, there is no explicit example where scribe 2 of 𝓟 ever mixed up shall and should, even momentarily. Oliver Cowdery mixed up the two modals fairly often, but usually only momentarily. Thus the odds are quite high here in 3 Nephi 26:9 that the 1830 typesetter is the one responsible for the variation. The critical text will restore the unexpected should, the reading in 𝓟.
Summary: Restore in 3 Nephi 26:9 the modal should, the reading in 𝓟, since evidence from textual transmission shows a fairly strong tendency on the part of the 1830 typesetter to replace uses of should with the more expected shall when referring to future events.