I bought a PT 24/7 Compact DS in .40 S&W a few weeks ago and it shoots just fine so far after about 200 rounds. Looked at the Millennium side by side and the trigger pull on the 24/7 was a lot sweeter.
First box was 180 gr. FMJ which kicked a bit in the small frame 24/7 C but the "ribber" grips helped a lot. Next was a couple of boxes of155 gr. Remington TMJ which were more pleasant to shoot while plinking at tin cans out in the desert.
I too was shooting 3" low at 15 yrd.'s but figured out my problem with some help.
I was using the standard Weaver weak hand thumb over thumb grip with all fingers wrapped around the pistol's grip. A friend suggested I try moving the weak hand thumb onto the "dimple" on the frame above the front of the trigger guard and placing the weak hand index finger on the front of the trigger guard, a style of grip I had never tried before.
This style of grip helped me get back on target with the small frame 24/7 and helped me manage recoil better as well.
I bought this gun for CC so size & caliber were important factors on what to carry.
I'm running Hornady "Critical Defense" 165 gr. JHP's as my carry rounds because you never want to go cheap on ammo you may, God forbid, have to use to save your or another's life someday.