Worth to power up this mons?

Venusaur for sure, he's one of my favourite attackers -with that moveset-.

Snorlax, yes, if it's your best one.

Lapras, I would better power up a defensive one...

All up to you

EDIT: Sorry, I mean my favourite Venusaur for attacking has Vine Whip + Solar Beam

No, no and no...IMO. MAYBE the Lapras.

I would sit on that Lapras and Snorlax candy until something better comes along.

1. Needs vine whip. You got access to rare candy so id make another.
2. Recommending to wait till after Christmas event if you are going to power it up. At 22-2300 it will take out 2-3 larger Dragonites. Im not one of the resident IV circle jerkers but 55% is low. Imo average and above average iv's(bottom 2 tiers) are not worth significant amounts of stardust or rare candy.
3. Maybe if it had BS and over 23-2400 cp. This one depends on your access to more and how many candies you got. But most likley you dont want to get in bed with this one too much commitment.

Venusaur if you want a defensive beast - wait for vine whip if you want an attacker.

Lapras - I'd use as an attacker as is and hold the candy incase you get a better defensive moveset.

Snorlax - yes unless you've got one with a better moveset.

if you like to attack gym then lapras for sure, your snorlax good for defending gym, I myself prefer to power up exeggutor better than venusaur.

Every single one of them.