there are 5 legendary Pokemon in leafgreen version
no but can in emerald!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Everything but preevolutions and legendaries.
gameshark
In total including event only Pokemon there are 10.
Abra, Exeggcute, Machop.
A ditto with (any Pokemon you put in the day care accept for legendaries)
Moltres, zapados, and articuno are the three main legendaries in leaf green/fire red
it dependz on if you already have national dex if so i have tyranitar lapras charizard/venusaur dragonite aerodactyl and i rock!!!!!
You can if you want to it's totally up to you i can say yes that's the best suggestion considering they overpower most regular Pokemon however if you want to prove yourself a true trainer you will not use any legendaries.
The catchable legendaries are the following: Articuno, Zapdos, Moltres, Raikou/Suicune/Entei *depends on your starter pokemon*, Mewtwo.
If you mean with the 3 legendaries, the legendary bird pokémon Articuno, Zapdos and Moltres: you can't catch them in Diamond. You can migrate or trade them, since they will be roaming Sinnoh in Platinum. You can also find them in FireRed/LeafGreen, HeartGold/SoulSilver and X/Y.
Yes, lower their health so it is 1-5, then put them to sleep or confuse. Use ultra balls to catch.