My 3DT (70% filter mod, powered by M8+C9, Symbio F tips) will be sitting proudly along side my Traillii.
It offers a more visceral bass impact, rumble and more pleasing decay (to my ear and to my preferences) without sacrificing a great deal of detail in the lows when compared to the Traillii. Traillii outclasses in its detail retrieval across all frequency ranges, but to different degrees, this gap being more clear in the mids and treble. The timbral accurary of most instruments on both is great (with Traillii a small way ahead because of its more revealing treble tuning).
Traillii is a superbly balanced in its presentation, while 3DT unmodded can be aggressively forward (for e.g. listening to Jeff Buckley on Grace with his massive vocal range can sometimes not be enjoyable at all) - thankfully a simple filter mod and tip rolling negates this and brings better balance.
Traillii isn't as source dependent as the 3DT. The 3DT demands a good source and shouldn't be judged until its heard through one.
The reason I plan to keep it as a complement is that it offers me greater bass impact and rumble, a more pleasing DD decay and a smaller and more intimate stage. It is also less of a loss if it were to go missing.
With a good source, perserverance and a willingness to experiment the 3DT competes at the kilobuck price point, easily. The Traillii is a distance ahead in most respects, but so is it's price point.
If you're reading this and thinking of picking up a 3DT, just do it.
The people in this thread are very friendly and together we can help you navigate the small obstacles to enjoying this IEM!