Sorry for answering too late, but I hope this answer helps as well:
Cloyster
Pros
-Brillent defence stat of 180
-Ability Skill link makes Rock Blast and Icicle Spear became super strong
-Atk stat of 95 was fine
-ACCESS TO SHELL SMASH
-Good one at cleaning hazard (Rapid Spin) and setting hazard (Spikes and Toxic Spikes)
-Stat was not bad
-Much much more better than the other two you’ve mention
-Could also be a special sweeper but however psychical was more better
Cons
-Super frail and bad Sp.Def of only 45
-HP was another big problem which only got 50
-Typing of Ice / Water was very bad defensively. :(
-Weak to Stelth Rock
Lapras
Pros
-Good movepool, definitely the best of the Pokemon you’ve mention
-Epic HP
-Defensive stat isn’t that bad
-Could be either Psychical or Special
-Best Base Stat against the Pokemon you’ve mention
Cons
-It’s abilities kinda sucks
-Again, same as cloyster it’s typing was very Frail
-Outclass by other water and ice type Pokemon
-Weak to stealth rock
-Not Actually a good Pokemon to use :p
Dewgong
Pros
-The ability Thick fat makes it take no neutral damage from fire type attacks anymore
-Could be in either Hail team or Rain team due to the other two abilities
-Good Special Defensive stat
Cons
-Bad bad typing
-Super frail
-Was outclassed by others in a lot of ways
Hope it helps!