In D2:ED I don't recall any Legendary quality equipment with 3 enchantment slots. Elder class equipment was added in FoV/DKS.
Random loot from chests or quest rewards in D2:ED did tend to be better at higher character levels, and a couple people recommended putting off learning Lockpick and then going back and opening all the locked chests before finishing up Broken Valley, etc. However, if you can get that far in the game without the loot, you probably don't need dramatic equipment upgrades. I opened most chests as I came to them, and didn't have any problems. There should be even less of an issue in DKS, with improvements to the loot from merchants, etc.
There is a limit to the range of possible stats on equipment, as well. If you are at level 13 and you are checking a chest where the random equipment requires about level 13, then you may get better stats if you wait until you are level 14 or 15, but at level 30 you are not going to get anything better than you would have at 15. There were a couple chests in D2:ED I left and came back for thinking that given the difficulty getting to them, they should be giving better loot than they offered in a couple reloads. However, being at a higher character level didn't help.
Early in the game I left the chest under the church that contains random creature parts until I finished the quest to get the creature. In that case the chest did contain much better parts than when I first tried it, but not any better than other random parts I had found by then.
In D2:ED the maximum level was about 37, without taking advantage of a couple places where opponents can respawn. Including the FoV section of DKS, I think that brings the maximum up to the mid forties, at least.
