Ok so good news and bad news about fighting Magick DKs, dual wield stamsorcs, and samplers.
The good news is most of their builds will be very similar or exactly the same so it's easier to fight against when you know what you're fighting.

Bad news is because of the meta it doesn't mean it'll be any easier. The best way to come up with a specific build against them is to research the meta or standard for each of those builds. You'll find a lot of similarites at the least.

Here's my advice for fighting each one.
For the mDK: I always use a staff and am a ranged mageblade which makes fighting the dks who can spam wings a major pain. I'd recommend either going dw or putting on concealed weapon as a melee spammable attack or just get really good at timing your attacks. You MUST conserve your stamina. if you dodge roll too much or block too much they will tear you apart with their burst by rooting you when you have no stam. Good news is they have similar concerns about blocking and dodge rolling as well. Time your bursts for when they run out of stamina.

For the dw stamsorcs: this is probably the hardest to fight against simply because there is no standard one way to fight against them. They have shields, heavy armor, lots of stam, strong weapon dmg, an ult that can make you useless, lots of maneuverability, and a constant aoe that does dmg and pulls you out of cloak if you use that. Honestly the best advice I can give for that is keep shields up, save stam, stack dots, and time your burst. You will be playing defensively in these kinds of fights (which is very bad to do for too long) so make sure to use los and watch for them to "see red' and burst them down

Finally for the stamplars: honestly these are the easiest of the 3 to fight. That said, they aren't always gonna be quick kills. They can heal better than anyone else with class heals and/or vigor and rally. The other nasty thing about them is they can purge. While most don't, it is a possibility. If they do purge you have to keep them on the degence so they forget about using it.
Stack buffs and debuffs on them, cc them in correct times (literally every time they use a gap closer use fear to keep them from ulting or ds) and then just dps them.

I'd recommend using heavy armor all impen if you aren't already and put some cp into tumbling to reduce break free cost. Other than that get with some friends who run those kinds of builds and just practice with them.

As a magblade against a MagDK you just have to time the burst inbetween wings. Havent fought a MagDK that has been able to keep up wings the entire fight. Not many extremely good MagDK's you run into open world as is so timing the spectral bow, meteor, fear is pretty easy. Always be sure to get a cripple in so they are ticking. They have unreliable heals if you can stay at range and keep the healing flame DoT off of you. Many are in heavy so make sure they are at about 80% health (with cripple ticking) when you burst or you may not kill them.