So we are just ending in Chapter 3 in Dragon Heist and i just hit Lv 3 with my Dual wield Fighter. My question is should i use my Treasure points to buy +1 Weapons before we get done with Dragon Heist or is a Bag of Holding a good thing to buy instead?
If you find that you are carrying close to your weight limit and want to carry more then a bag of holding might be worthwhile. I run into this a fair bit with lower strength characters.
I've never actually worried about the weight factor of carrying gold so that tends not to be the reason to have a bag of holding in the first place.
Bag of holding is on the evergreen list and will always be available ... you can get it anytime. If you don't have weight or carrying issues now then don't bother with it since you can always pick it up later.
As for +1 weapon ... might be worthwhile for hitting targets with resistance to non-magical damage. Other than that it isn't a big deal.
Another bit of info to consider. There are only a handful of monsters resistant to non-magic weapons at tier 1, but that number really ramps up for tier 2. I can understand not wanting to just have a +1 weapon, I personally prefer something different like wounding, flame tongue or warning, but if you don't have those unlocks a +1 is ok. And you may be able to trade it later on if you get access to a better weapon.