Where do you find the 3 legendary dogs if I have venasaur ?

Here's the three legendary dogs with the starters.
If you started with ___ you'll find this legendary dog anywhere in the region.
Bulbasaur: Entei
Charmander: Suicune
Squirtle: Raikou

As with the other two, you'll have Trade from another game.

You find one of them wild in random grass areas and if you have venasaur I believe you find Entei

Ok, after the Elite Four one of the legendary dogs will be roaming the grass of Kanto. You will have to encounter it at random. The one you get is determined by your Starter so if you have Venusaur, you will get Entei.

You will get an Entei, the fire dog.
It will be random when you see it but they will flee very fast.
If you have a wobbofet it has the ability shadow tag wich prevents escape.
Than just weaken and then catch.

You can catch all three in 'Pokemon Colosseum' and Trade them to your game. (It might only work with Ruby and Sapphire, so you may need to trade to them, then trade to fire)

In Fire red, If you picked Bulbasaur as your Starter, you will be able to encounter Entei, the fire dog.----------------------------------------------------
If you want: then start with:
Entei (fire) Bulbasaur
Suicune (water) Charmander
Raikou (electric) Squirtle

You find Entei if you got bulbasour for your Starter. Entei is found all around Kanto

Bulbasour:Entei
Charmander:Suicune
Squirtle:Raikou

You can only get Entei if you chose venusaur. And it will be found anywhere in the Kanto region (not the sevvi islands)

To get the other two you wouldh ave to start a game with the other 2 beginners.

Bulbasaur - Entei
Charmander - Suicune
Squirtle - Raikou

They just come out of random areas.