Where are the last 3 gyms in Pokemon Blue and how do you get in?

i don't have blue, but I do have lg so I should be able to know what to do

Step 1. Get Master ball in silph co. place and the 6th gym will openStep 2. Surf from fushia city to cinnabar island and get that key in the Pokemon mansion, and unlock the door to blaine's gym.Step 3. After returning from three island, fly from vermillion to viridian and go to the gym there. The door is unlocked now. You can battle Giovanni.