Yeah I wouldn't bother with 3* at all, knowing what I know now. The quick boost in power may be tempting but 4* weapons will be much more useful in the long run. With tier 2 weapons giving anyone who can equip them a skill, and then the eventual upgrade to tier 3 elemental weapon that will give on-element wielders a massive boost. Plus some 4* elementals are actually preferred to their 5* counterparts due to choice of skill.
EDIT: Oops, your last post popped up while I was still typing this. The nice thing about the void weapons, like that spear, is that they don't really hog up many resources that you'd need for non-void weapons. You should be able to upgrade both to your heart's content without a lot of struggling for materials, I think.
EDIT 2: Also, it seems that you're like me and somewhat desperate to get stronger quick. If you can help it, I would try to resist that urge and try to think more long term. Do a little bit of upgrading, sure, but no need to fully outfit your group with the best you can manage, when in a short while you will have better things available that you'll wish you had the money and materials for. Just my opinion, though.
