Verified answer

Feebas is found fishing on mt. coronet with the super rod. It is EXTREMELY rare. About a 1% chance of finding one. Mespirit and cressellia. I have a strategy: First just find it to beat it down and paralyze it. Then get a lvl. 49 wobbufet and use repels if you get in the same area they are in. It's shadow tag will prevent them from running. You can use as many balls as you like but save when it's in the red as when it's moves are used up it will use "struggle" and that will make it knock itself out. Flying to the location they are at will make them move to a random place. The trick is to find a path with multiple routes so it won't just move between two locations.

There is no end all strategy to get in the same place they are. Only suggestions others like myself have used. It's a matter of luck and figuring out where they will go if you move to X.

the best way to find Feebas is to fish on ALL the water with an old rod. The best way to catch Cresselia and Mesprit is to put them to sleep (after using mean look) then use ultra balls on them, the best way is to press A repeatedly after the pokeball turns yellow for a brief second after the pokemon goes into the pokeball.

Feebas is catched at mt. coronet where the lake is where it is foggy you need DeFog.

Easiest way to catch Mesprit is with a master ball and cresselia to.

well..

Mesprit & Cresselia

you can use meanlook with crobat

Feebas:

it may take hours

here is what you do:

go to mt coronet basement ( large lake)

then you fish in every single tile ( use a pattern)

at some point you may find a feebas

but here is another option:

if you have wi fi i can give you one

Mesprit- Walk in grasses in Sinnoh after meeting at Lake Verity

Special: After the events on the Spear Pillar, you can find Mesprit in the cave in the middle of Lake Verity. It will run away after you talk to it, so you will have to chase it using your Marking Map Poketch app. (Only one.)

at Level(s): 50

Cresselia-Special: After obtaining the National Dex, talk to the boy in Canalave City and then talk to the sailor to sail to Fullmoon Isle. Talk to Cresselia there and it will then run away, so you will have to chase it using your Marking Map Poketch app. (Only one.)

at Level(s): 50

Feebas-Found in Mt. Coronet, by fishing with a Super Rod, but only in four random tiles.

The easiest way to catch them, is to switch from city to city with the pokemon locater on your pokenav. As for the other two, I don't really know