the only way is to trade from Pokemon sapphire or ruby.
island 3
Venusaur
emerald isn't on there
There is no Mew 3.
There are 7 catchable rare Pokemon in leafgreen however here's the catch, there are 7 but you can only catch one of 3 legendary dogs Raikou/Suicune/Entei it depends on what starter Pokemon you chose. Also there are 3 event only Pokemon you can get making the total to actually 10 in all.
All the 150 Kanto Pokemon exist there as well as a good bunch of Pokemon from Gen 2 and 3. You can't get all of the Pokemon without trading with somebody with Pokemon Emerald.
You can find Pokemon from Generation 1 and 2 there is one Pokemon you can get from Generation 3 but only if you have been to a Nintendo event (it's Deoxys btw).
In Berry Forest on island 3
migrate from firered and leafgreen
There is only one
You have to trade them from ruby, sapphire or emerald.
you can get them by either filling all your pokedex in emerald and talk to professor birch and then migrating them or AR.