After you resurrect S. America and revive everyone in the Miller's house, you continue on to Tower 3.
Tower 3, 1F
Go north. Walk along the arrow platforms after they move into place. From the large center platform, go to the right, jumping over the gaps. Go north and down the stairs to reach a downstairs area where you find Magirock. Go back upstairs. Go across the arrow platforms near the top of the room, but hold down the button to run across, because they are going to fall. If they all fall and you can't get to the platform in the middle, go down the stairs in the upper right of the room.
When you get to the area between the falling arrow platforms, go up the stairs. You see a chest to the right. You can just walk through the stones to the right to reach that chest. It contains an S.Bulb. Now go up the stairs to go to the next floor.
Tower 3, 2F
Go up the stairs to the left. Run past the wall spears when they go back toward the wall. At the bottom of this area, you find four switches, and some arrow platforms to the right. The switches make the arrow platforms move. Press a switch again to stop the platform from moving. You don't need to push the second switch because the second platform is already where it needs to be. Press the other switches to make the arrow platforms form a diagonal line downward toward the ledge on the right. Walk across the arrow platforms to go to the right.
Go up the stairs and go past the wall spears. Up above, a Guardner appears. It can cast sleeping spells and pull you into a vacuum. If you get pulled into the vacuum, you are sent back to the entrance of the tower. Go down the stairs south of the Guardner, then go west and north to reach the next set of stairs.
Tower 3, 3F
There is a Cadet nearby. Kill it for experience if you wish. Go to the left or right and jump over the gap. Go north and quickly run past the Guardner so it can't cast a sleep spell and send you back to the entrance. Go past the wall spears, then continue north and get past the Cadets to reach the door up above. Approach the door and it will open.
Go north and then jump left across the gaps to a narrow ledge to the left. Open the chest there to get a Sleepless Seal. You can equip it to prevent the Guardners' sleep spells from working on you.
Now jump east across the gaps to find Magirock in the southeast corner of this floor.
Go to the north end of the room and press the two switches to open the north door. A bunch of Huballs come out of the door. Fight them if you want the experience, then go through the door.
In this room, go to the top of the room and then down the path to reach a cross-shaped area. The Guardian sends a High Cadet to attack. You have to hit the correct one. The correct one is whichever one goes behind you. Hit the Cadets until you defeat them. Then go north through the door. Africa is resurrected, as are the people who live in the Domicile.
You can reach a secret area if you go as far north from Tower 3 as you can, BUT you won't be able to do anything there until you have completed all five towers, so just continue on.
You can go back to town if you want, or just go east from Tower 3 to reach Tower 4.