Why won't my car start?

I have a 2000 buick lesabre custom with a 3.8 engine and recently it won't start, when i first noticed this problem it took me a few tries but it started then later that day i got stranded at the mall i tried getting a jump(no luck) ended up having it towed now since then i have had a new starter installed, had my brand new battery charged and checked, and changed the starter relay(which on my car is a little box shaped plug under the hood in the fuse box), checked for corrosion on the battery it's self and the cables and there is none to be found, mind you all of the lights and other electronic gizmo's on my car are working and when i go to start the car i hear a click or a low and slow whirring sound as if the engine is trying to turn over but won't and with the new starter i had it tested to make sure it was working and it worked just fine when tested but when installed it won't engage the flywheel now it is the same size as the old one that was taken out (which i found out after i bought the new one was fine) we tried installing it with 2 shims still nothing, it makes the click and that's all or the low and slow whirring sound or sometimes no sound at all now i was told there are no broken teeth or cracks or holes or dents in the flywheel so what could it be? oh and i don't know if this helps but my car has a voltage reader on the dash and it is reading at an 11.3 so can anybody please tell me what could be the cause of this disturbance