How does 3 super mutant appear inside the building, when there is walls around the settlement and the entrance is heavily defended and also with fences to prolong the entrance?

Character was at Sanctuary and the shooting across the bridge could be heard, so he heads over there and find super mutants coming down the hill but there are also about 3 inside the building. Can they pass through the high hedgerow bushes even if the character can't?

Yes.

Enemies can spawn anywhere inside settlements so it doesn't do you any good to build walls or any other obstacle to keep them out.

http://www.gamesas.com/topic/1578214-11-machine-gun-and-5-laser-turrets/ I started about this exact subject at this exact location.

I always fast travel to Concord and walk to Red Rocket because it seems to trigger attacks sometimes when I do that and I like to watch my fortifications in action.

Edit: I've also noticed that if I'm wearing Power Armor and I'm at maximum weight capacity but still able to run and I get out of the armor and am now weighted down so I can't run, then go to a workbench to break down weapons, that will trigger attacks.

You can still hear what's going on in the background while working at a bench and I'd say about 90% of the time when I go to a bench while I'm carrying too much I'll get attacked. But it only seems to happen if I was in Power Armor and step out, not if I take inventory from a companion and then become over-weighted.

I like to set up a lot of defenses and watch the battles.

16 Heavy Machine Guns on the roof. 4 Heavy Lasers on the house on the roof, a Shotgun on the landing and a Heavy Machine Gun in the main bay downstairs.

I sometimes pause to watch the show, often I don't.

I have the Shotgun covering the landing and stair area and it can't see most of the rest of the area. So if it starts to work out I drop what I'm doing. A Super Suicider once made it onto the landing. I actually took a tiny bit of Rad damage. That's as far as anyone has got.