Originally posted by Therk:

Looking for the 3rd boss summon

You MIGHT find a random snowboime
You MIGHT find an Ancient ruin in snowbiome
You MIGHT find a boss summon "locator" in a Ancient ruin

so far i found several "ruins" but not a single "boss location stone" that show where can i summon the boss

is there an easy way to find that ?
dont want to cheat

The Moder location stones are always in a Stone Tower or structure of some sort. Large Mountains have a pretty good chance of one somewhere. I feel you, though; I had one world where I had to look damn near everywhere and Moder ended up being more than halfway from spawn to the south edge of the damn map.

For Yagluth (once you get to that point), the location stone is always next to a stone structure of some sort, but not stone towers (think Stonehenge-style structures). The actual Yagluth arena is also incredibly obvious to spot from a distance (by far the easiest to identify from range out of any boss spawner). If you see five massive rock towers all in a circle, with one rock tower slightly shorter than the rest, that's a Yagluth spawner.