1. If it snows tomorrow, I'll not go hiking.

2. Because it snows tomorrow, I'll not go hiking.

3. Because it will snow tomorrow, I'll not go hiking.

(Which one is correct of the two, #2 and #3?)

3 is correct, I guess, but better phrasing is this:

Because it will snow tomorrow, I won't go hiking.

Of the two options, #3 is considered to be the grammatically correct sentence. The reason is that the use of "will" (future tense) is appropriate for talking about a future event that is anticipated but has not yet happened. In this case, the speaker is stating that they will not go hiking if it snows (in the future). However, it is worth mentioning that sentence #1 is also correct in terms of its structure, but it does not express the cause-and-effect relationship between the snow and the decision not to go hiking.