There is a lot of good advice here ... the most insightful comment is that Molle Packs are designed to be used with Military Equipment and often lack the weight distribution and torso adjustability of civilian counterparts. A 3 day pack is my most used format of pack and like many here I have lots of packs. If I were putting on a Kevlar vest under a military smock and using a webbing belt system over the smock for ammo, knife, water bottle and stainless mug, mini stove and rations and water purifier pump and lightweight down jacket and first aid kit ... the back kidney pouches are fairly full and the trick to carry comfort for the pack is to get the torso size right so the pack sits on the kidney pouches and the webbing belt used with a hippo pad can take the weight off the shoulders in the same fashion as a civi pack with a good hip belt. You don't really want a big padded belt on the pack as there is no room to use one and the frame of the pack matters little as the thickness of the Kevlar vest acts like padding and frame comfort cannot be felt in the same way as a civi system used with a tee-shirt.
The point being that what works well in a military "system" is not so good on a stand alone use of a pack where you are not using webbing and want a fully adjustable back system and good padded belt to assist in stability and weight distribution and ideally good back ventilation to stop sweat build up on your back.
I find that the best 3 day packs which are to be used as a stand alone pack are civi packs which are of the non molle type.
If you get a Molle pack suited for stand alone use it is not good for military use because the first requirement in a military role is that if you ditch your pack - you need to have a webbing system you can live off and continue to fight with - and ditching your pack is very likely necessary if you get into a contact and need to move fast and present as low a target profile when prone as possible.
So once you factor in the differences between the two systems - a good Molle stand alone pack is a bit of an unusual "spec" and unlikely to be an "issued" bit of kit ... like as not you will find more comfort and less weight and better torso adjustable systems on a civi pack.
For military use I use Arcteryx LEAF packs ... for civi use I go light-weight for a 3 day pack and use Osprey or Berghaus or OMM.