You’ll get it as part of the story
Although you’ll start off God of War Ragnarok with the two main weapons from the first game (the Blades of Chaos and the Leviathan Axe), you’ll also gain access to a third core weapon type: the Draupnir Spear.
How to unlock the Draupnir Spear
You’ll have to cruise through God of War Ragnarok for quite a bit with just those two aforementioned weapons, but roughly past the halfway point of the main narrative, you’ll get the Draupnir Spear.
You’ll know it’s coming up as the quest is called “Forging Destiny,” located in Svartalfheim. All you need to do is scrape a few reagents off the rock in the quest area, then go down into the depths with Brok, and you’ll get the spear.
Using the Draupnir Spear
To select the Draupnir Spear, you’ll press down on the d-pad. Think of it like a “neutral” type weapon that doesn’t utilize the fire of the blades or the ice of the axe. In other words, it’s generally great for every situation, unless you need to whittle down a shield off an enemy (which you’ll do with the opposite type of element). One of the best ways to wield the spear is to toss an infinite amount of projectiles (hold L2) at enemies: which is also the solution to a lot of in-game puzzles.
Also, keep in mind that all three weapons do not share cooldowns for light and heavy abilities. Make it a priority to swap between each weapon and constantly unleash your cooldowns. It’s going to be hard to get the Draupnir Spear in that rotation having worked with the other two for so long, but just get in the habit of using the spear, and you’ll remember when it counts.