Question

How do you get Mudkip,Torchic and Treecko in HG/SS?

Answer 1

Well after you have defeated Red, you can obtain them after visiting Steven Stone in Saffron City.
He will compliment you, ask you what colour stone you'd pick and then
provide you one of these Pokémon based on the colour. As with all
starters, they will be at Level 5.

Source

Answer 2

You can get them after you beat Red on the summit of Mt.Silver. You then go to the Silph Co. building in Saffron City, and he will offer a stone to you.
The red stone is Torchic.
The green stone is Treecko
The blue stone is mudkip

Hope I helped!