UNITY_HARDWARE_TIER3 is set when compiling shaders for GraphicsTier.Tier3.
Note: Graphics tiers are only supported in the Built-in Render Pipeline.
Unity generates tier shader variants if your shader source code includes #pragma hardware_tier_variants, or if the Tier settings for your project are not all identical.
For more information on Graphics tiers and shader variants, see Graphics tiers.
Additional resources: Graphics tiers, TierSettings GraphicsTier, Graphics.activeTier, EditorGraphicsSettings.GetTierSettings, EditorGraphicsSettings.SetTierSettings.