how is the challenge rating judged in pathfinder? cr3 is what, versus 1 PC of 3rd char level? versus 4 characters of 3rd level each?
https://www.d20pfsrd.com/gamemastering/#Table-CR-Equivalencies
Add up the XP of all the monsters you're including
Total XP should equal the amount shown on this table, where the CR is equal to the party's average level.
If you have 6 players, you add 1 to the party's average level. So 6 level 1 adventurers have an average level of 1+1=2.
Black Bear has XP of 800, which makes it a CR3 encounter when alone. That's 'Challenging', according to Pathfinder (APL+1):
Difficulty Challenge Rating Equals
Easy APL –1
Average APL
Challenging APL +1
Hard APL +2
Epic APL +3
A Black Bear could definitely kill a PC in one turn if you got lucky.
BUT
The trick to good GMing is to look at the skills and abilities the bear has. The bear can swim very well. So, you can use this creature anytime the PCs try to swim across a river, for example. In the middle of their swim, a Black Bear appears, and attacks the party while they're swimming.
Also, the bear is fucking FAST. You could use this monster effectively to chase down a wounded PC, grapple it, and kill it.
So yeah, if you make the fight fair, the Bear's going to get fucked. The trick is not to make the fight fair, by utilizing the creature's abilities/skills.
Also, as a 'Challenging' encounter, the Bear is not supposed to TPK the party. It's only supposed to stand a decent chance at killing one of them, perhaps. And with 3 attacks per round and auto-grapple, it could definitely do that.