Which eartip to use with Westone UM3X?

Yes, tri flanges or tri flanges modified to bi flanges will fit fine but I find that they cut off the treble a bit and make the closed-in sounding UM3X sound even a bit more congested. Some don't find this to be the case though.

I purchased some large size black T-100 series complys second time around and they don't show dirt as bad and fit even a bit better for me. Sound great too.

I've been using the Klipsh ovals.

go get a um56 and be done with tips. it will save you in the long run and not to mention it sounds better than any tips i tried.

Re: Klipsh tips -- fwiw, I've only used the Klipsch and the Comply that came with the UM3X -- so, with that said, I much prefer the Klipsch -- to me they sound better, feel better, and will last longer. I now only use the Comply foams on a plane, for the additional isolation. Right now I'm using the Klipsch large double flange. Given that we're all built a bit differently, the ideal is to try everything you can, and figure out what happens to work for you.

As for me, I happened to have the Klipsch, and they work good enough that I don't feel compelled to keep getting other kinds -- that said, I'd would still consider other kinds of tips if they happened to magically fall into my lap.

I know you can get 'fit kits' from Westone (which I'm not interested in b/c they don't then also sell specific tips individually) and from Shure -- the Shure fit kit might make sense. I'm not sure if Klipsch offers a fit kit.

Shure black olive foam tips should fit the UM3X. They are more durable than Comply.

I've tried the Shure black olives on the UM3X, and the tube is ever so slightly too small on the UM3X for these to stay snug (yes, the tips were brand new and unused). I pulled the UM3X out of my ear, but the tips stayed in. They're also firmer than the gray foams that come with the UM3X, and put more pressure than I like on my ear canals.

These fit very nicely, and are easily the best silicone eartips I've tried (compared to the Ety tri-flange and Sleek bi-flange). They're my second favorite after the gray foamies that came with the UM3X, which are hands down the most comfortable tips I've ever used. The Klipsch bi-flanges are actually really nice as well.

Yeah, the Comply's are really nice.

+1 for Shure Olives. They're simply a great option and vastly more durable then the Comply. I do agree that the tips don't go not tightly. Mine stayed fine, but I'm not exactly shoving the tips way in my ear either. I had no issue though with it.

Etymotic tri-flange would probably be a nice option too, but I'm not a personal fan of the flanged ones. They always seemed to dig into my ears and were annoying.

The small Shure is kind of like the medium Comply.