As far as I can remember, the timeline of the various shows pretty much lines up with when they aired, that's the sequence of events in this universe.
I watched Defenders without having seen Luke Cage or Iron Fist, and pretty much managed to follow what was going on. Having then gone back to watch the shows in order, in terms of understanding the plot of Defenders, Iron Fist is the important one - you'll already know Luke Cage from S1 Jessica Jones, and I don't think anything from his show ties into the plot of Defenders, other than the existence of a couple of supporting characters perhaps. Daredevil gets name-checked once or twice in Luke Cage, but that's about it.
Defenders directly ties into the beginning of Daredevil S3 - which from what you say you've already seen? So you'll already have seen how DD S3 picks up in a different place from where S2 left off - other than that, the events of Defenders don't really figure into the plot of that season, so you aren't missing much. Defenders is fun to watch in terms of seeing all these characters actually interacting, but the storyline isn't the best and the plot is all over the place. Luke Cage is a moody, broody kind of show - has some really entertaining characters, but can be very slow, and Luke himself is not the most engaging protagonist ever. Iron Fist is...I would say the bubblegum offering of the Netflix Marvel universe. It isn't a great show, Danny Rand is dumb as a box of rocks, but I actually found it a lot more fun to watch than Luke Cage. Not least because they remember to switch on the damn lights from time to time! And again it has some really entertaining characters.
So...you aren't missing much if you decide not to watch, but then again you might have some fun with them if you do!
