Is this "Timberwolf" Balanced?

Question

Based on the dire wolf an a twig blight, this creature is a essentially a large wolf-like construct made of wood, held together by arcane magic.

Timberwolf

Large Construct, unaligned

Armor Class 14 (natural armor)

Hit Points 37 (5d10 + 10)

Speed 50 ft.

Damage Resistance psychic; bludgeoning and piercing from nonmagical weapons that aren't adamantine

Damage Immunities poison

Condition Immunities blinded, deafened, exhaustion, poisoned,

Damage Vulnerabilities fire, slashing

Skills Perception +3, Stealth +4

Senses passive Perception 13

Languages –

Challenge 3 (450 XP)

Fragile Frame. When the wolf dies, it falls apart into non-magical wood. Indistinguishable from broken branches.

Keen Hearing and Smell. The wolf has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Pack Tactics. The wolf has advantage on an attack roll against a creature if at least one of the wolf’s allies is within 5 feet of the creature and the ally isn’t incapacitated.

Pounce. If the wolf moves at least 20 feet straight toward a creature and then hits it with a claw attack on the same turn, that target must succeed on a DC 13 Strength saving throw or be knocked prone. If the target is prone, the tiger can make one bite attack against it as a bonus action.

Actions

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage.

Would this creature be a level 3 challenge rating? Are there things that I missed in the description the make the creature too OP or UP? Is Slashing damage a good vulnerability? What level should my players be before I introduce this creature in a pack of 3-5? for that matter, how many of these creature would be a good number to introduce the creatures in a first encounter?

Answer 1

It seems too weak.

From the DMG p. 273 onwards, if you make the math:

For defensive CR:

It is not resistant to Slashing, on the opposite, it is vulnerable. As it is resistant to Bludg. and Piercing, I would say it doesn't change the CR, but by strict DMG ruling it probably would have even lower effective HP. With the HP you got there, it would be a CR 1/4 monster. As the AC is only 1 point above the usual 13 for a CR 1/4, it stays at CR 1/4.

For offensive CR:

The Pack Tactics means +1 Bonus on attack. The Pounce increases the DPR by 1 for Claw, which is still lower than the (effective) 11 from Bite (due to the prone effect, +1), so that is our DPR. That's a CR1 monster. A CR1 monster would have +3 Bonus on attack, but your Wolf has +5, so it becomes CR2.

Final Rating:

\$ \frac{1}{2} \left(2 + \frac{1}{4}\right) = 1.125 \$

The DMG says that you should round it up, but honestly it seems just way too weak even for a CR2 monster. Honestly, this seems a CR1 creature, but since it is so unbalanced between Defense and Ofense, you shouldn't put it against a Level 1 party, as it could potentially kill their Wizard. Still, it's clearly weaker than an Ogre, for example.

Your questions at the end are... lots of questions. It is too UP for a CR3, Slashing Damage is a vulnerability that decreases the effective HP by alot, as stated in the DMG, so no it's not good. The next questions are about balancing encounters, which is a totally different question and is answered in p. 81 from DMG or p. 56 from Dungeon Master Basic Rules.

Answer 2

Vulnerabilities

Generally, vulnerabilities are an extremely rare sight in 5e (see this list of resistances, immunities and vulnerabilites of MM monsters). This is because it effectively decreases the hit points by half, once the players have discovered this vulnerability (I generally narrate vulnerabilities, resistances and immunities, since I believe it makes sense - a fire elemental won't be bothered by a fireball, while a scarecrow will be more or less a walking torch).

Anyways, I recommend being careful with vulnerabilities, and giving a monster two of them even more so. In the whole Monster Manual, there is only one creature with more than 1 vulnerability, the Ice Mephit, page 215.

Creature Type

You seem to imagine this as a plant-based creature. If that's the case, I recommend classifying it as a Large Plant, not Construct - those are generally more something such as a Golem, i.e. man-made constructs.

Keen hearing and smell

This makes sense for the dire wolf you've based your creature on, and you can, of course, also give a magically animated bunch of twigs improved senses (because magic). However, it feels wrong thematically, so I'm not sure I'd leave this trait as it is.

Hit points

The DMG features a table on page 274 that lists "quick stats" - these mention 101-115 hit points for a CR3 monster, while the 37 you used are more in the realm of a CR 1/4 monster. A level 3 raging barbarian with a greataxe can easily deal 1d12 + 5 = 6-17 slashing damage in a turn - double that and your average level 3 party won't need more than 1 round to finish this creature off; the barbarian might even one-hit kill it if he crits. That's not what medium encounters (which a CR3 monster should be for a level 3 party) are supposed to be like difficulty-wise.

Anyways, I'm not quite sure what made you declare this a CR3 monster - the dire wolf, which your statistics are largely based on, isn't more than a CR1 monster either.

Conclusion

This monster is most certainly not CR3. With the current stats, CR1 or at most CR2 is more appropriate - I recommend having a look at the table in the DMG mentioned above for guidance on how to choose your monsters stats.