I have my fighter wielding a 2h sword with AC of -10, Keldorn with -8 and Jaheira with -11.

In the underdark I equipped him with the red dragon shield, drow plate mail and his AC went down to -16!
My group is pretty much unbeatable now, (the last hard fight I had was against the the Gauths and thingys in the eyless quest (chapter 2) and I'm in chapter 7 now.

So, are these figures better than normal or does everybody feel the game easing after chapter 3?

Uhhm my ranger (stalker)has a ac of -10

And remember they cant ware anything better than leather armour

My Kensai can't wear armor, but I have his AC down to 0.
Minsc is -7 when wearing Kraag scale armor, helmet and shield.
Jaheira is -9.
Not as impressive as yours, you must be farther along than I am. Those AC bonuses on level ups are nice

My Paladin, with no shield and wielding the HA is AC-9 in Chapter3.
- red dragon armor
- helm of Balduran
- RoP +2
- guantlets of dex

(Minsc and Jahiera are -8 and -7 with shields.)

I got my fighter down to -15 in the underdark. I'm back in the daylight now though so I think I'm up around -13. I gave up a point or two to use equipment that bettered my resistance to magic. Not much can hit me with physical attacks anymore (or at least not frequently in a battle) so I decided to concentrate on those annoying magic users and try to take away some of their effectivenes

No. The tougher you get the tougher they get. However I had a monk, and so I changed my strategies in later stages to pretty much walking around solo scouting and doing damage. With high magic resistance and "ludicrous speed" I only needed the others for some of the mega battles. I enjoy it when it's not too impossible anyway. I mean, reloading 10 times to kill something isn't exciting - it's just boring.

Actually I don't find that the enemies have kept up with my a.c. With a few exceptions my Fighter is capable of wading away in front of my party without too much danger, not something I would have tried earlier on in the game. Sure there are some big things out there that can kick my but, though more often its magic users that mess me up - hence abandoning ac for resistance. Unlike your monk my resitance is pretty low without the help. Then again I could be wrong... it could be that my fighter is just killing things faster lately, leaving enemies less time to mess me up (I recently started playing around with dual-wielding axes when before I used to be a strictly axe and shield kinda guy).