If all you're running is 3 lights on a tree, then perhaps MyDMX is over-kill for now. Just use a cheap controller and a dimmer pack and power them on/off via setting the dimmer pack channels to switcher mode.
But if you stick with MyDMX, then on the plus side, it will grow with you. Also, for those who don't mind a bit of a learning curve, the work you put into MyDMX makes it easier on you later on, so for the price, it's in my opinion a better way to go anyways.
As far as your valid issue about "what lights have sound active in DMX mode" query, the best way to answer that question is for you to cruise the ADJ web site, find the fixtures you're looking at and then download manuals. Inside the manual is a listing of DMX attributes. That listing will show the fixtures in various channel modes(if they have multiple modes) and what features are available. Not all lights that can do sound active in stand-alone mode have had sound active implemented into their DMX attributes.
MyDMX can shut off a feature if your fixture is in DMX mode, AND IF the feature exists in DMX mode that you are trying to enable/disable.
Now, since you're into stage lighting that can be used in sound active mode, let me give you a fixture that I have: the 64 LED Pro. This fixture is a Parcan 64-style light with RGB color mixing(no gels needed, make any color you want). The fixture also supports strobing and also sound active, although I'm still playing with the sound active mode stuff to refine it, especially since I am using that in part of a show I am working on.
But, even though you are doing a "karaoke spot"(my live sound production company offers karaoke services), you might want to see about a bigger area wash light, such as the Mega Panel or Punch Pro, so download those manuals as well. I think the wider area coverage will benefit you better.
As far as other lights, are you intending to shoot stuff into the audience or keep it on the stage/performer? The reason I ask is because you may want to consider a pair of centerpieces, or a centerpiece and a scanner perhaps.
I find the combination of videos and manuals really helps me start the decision making process. However, in my case, my needs have become more and more complex and demanding, so I have to rely on additional help to ensure I'm going down the right path.
One thing you do want to do is evenly load your t-bar and stand. You can also mount 1 fixture on the stand itself in order to keep the T-bar itself reasonably balanced. You're also going to want to invest in some safety cables and come up with a way to ensure your lights don't slip off the T-bar. You also have other choices to make. You can use the included bolts to secure lights to the T-Bar, but use your own judgment if you're going to do this or not, which comes down to safe transport. You can get Super Clamps or C or S clamps to secure your lights as well. For me, I prefer the ADJ O-Clamps, as they are non-marking and I leave them on the fixtures. If you plan on mounting something on the stand itself, DO get the o-clamps so you don't mark up the finish. They aren't that much if you buy them for the amount of lights you have.
In short, you have a lot of research and work to do. I also don't think you're done asking questions either. My suggestion now is to check out lights that you think look cool or address your application needs and then download and read the PDF file for those products. After you've narrowed it down for a bit, chances are you're going to have questions again, which would be the right time to ask questions in regards to fixtures.
In the meantime, since it appears you're going down the MyDMX route, you'll no doubt have questions about that as well. Those questions you shouldn't wait for.