A normal werewolf is, as you yourself have noted, definitely out of the question for a usual party four of L1 adventurers. With its multiattack, it has a good chance of knocking out even the beefier members of the party each round. Add to that the werewolf's damage immunity to non-magical attacks from non-silvered weapons of normal damage types (Bludgeoning, Piercing and Slashing). You can end up with a situation where your PCs can't do anything because they get knocked out too fast, and even when they don't, only casters can hurt the beast.
Simple Solution: Tone down the werewolf
The easiest way to adjust monster levels in DnD 5e is to use the existing stats of a lower-level monster. In this case, I'd recommend using the stat block of the thematically similar 1/2 CR monster Jackalwere (DnD Beyond) paired with two wolves as minions (DnD Beyond). Take out the Jackalwere's Sleep Gaze ability, because it's definitely a Jackalwere thing, not a Werewolf thing. The Pack Tactics ability, while not a part of the Werewolf's stat block, can stay, because it makes good enough sense for a werewolf preying with their wolf buddies.
The inclusion of two wolves as "sidekicks" helps to make the encounter feel more like a boss encounter (1/2 CR isn't enough for that on its own) and also adds a desirable element of progress to the combat: taking down enemies is more satisfying than simply whittling down one bigger foe's HP pool. The Jackalwere shares the Werewolf's immunity to non-silvered, non-magical attacks that deal one of the three usual damage types, so the inclusion of minions gives your non-magical characters a chance to be helpful despite the main boss being immune to their attacks.
However, you should give your party a silvered weapon. I wouldn't advise arming them all with silver blades, because the moment everyone has a silver weapon, they stop feeling special and cool.
If you plan to use Werewolves later in the campaign, it might be helpful to describe the reskinned Jackalwere as scrawny, sickly or otherwise weakened. This way, when you get to the actual CR 3 werewolves, the players know not to expect them to be as weak as the Jackalwere.
The resulting encounter with 2 Wolves, 1 Jackalwere still counts as Deadly for a party of L1 characters --- appropriate for a boss encounter. Try not to drain their resources too heavily before the encounter, if you don't want to have character deaths here.
Bold (and Dangerous) Solution: Arm the players up
If you really want to use the actual CR 3 statblock, this is another way you can try, but be warned: this is a solution I really don't recommend doing at L1 for beginner GMs. That said, I think it's still worth documenting for future use:
Arm your players up with magic weapons, wands or other tools that make taking out the Werewolf possible for a party of L1 characters.
I have done this myself: the party was facing a monster above their usual paygrade, and I gave them a wand that could kill the monster. It worked, kinda. The balancing is fickle: the encounter was resolved quickly by unloading the wand into the monster, and you'll want to avoid that because it feels like wand did most of the work, but you also want to avoid the item(s) given being too underwhelming to actually help your party beat the beast.
You also want to give every character something meaningful to do during the combat, which is tricky if success is too heavily rooted in the use of a few magical superweapons. Finally, there's the issue of "rocket tag": if the party has a magical item that can easily slay the beast in two rounds, but the beast is powerful enough to slay the party in two rounds too, the encounter will be largely decided by the initiative roll.
The balancing act is so hard that I would recommend you to go with the Simple Solution for now. Experiment with the magic items later and cautiously, once you get a hang of the level of challenge your players can handle.
Different Solution: Level Ups
Design the adventure so that the party is no longer first level when they encounter the Werewolf. A well-rested L2 party has a much better chance against one than a L1 party does, and for an L3 party you probably want to pair the Werewolf with minions just to make sure the combat is still challenging enough to feel like a boss battle.
The easiest way to do this is to grant the party levels at appropriate points during the story leading up to the Werewolf encounter, eg. minibosses or other tense encounters. You can also use the traditional XP approach and give the players enough encounters and other XP-awarding challenges to level them to L2 or L3.
Just remember that the damage immunities of Werewolves are still very punishing to non-magical melee classes, so hand out a silver weapon when using this strategy, too!