You should allow for a couple of rainy days during the trip.
Literal
Trip-during [time-に] 2-3 days rain [subject-が] fall [nominalizer-こと] [object-を] consideration [in-に] put-in [preparatory-ておく] [conditional-と] good.
考慮に入れておく chains three patterns: 考慮に入れる (to take into consideration), ~ておく (do in advance), and ~といい (it would be good if). ２、３日 (2-3 days) is a typical Japanese way to express 'a few days.'