I own the P6 and the OB 6 and while they each of course have a distinct charm, I still often question if I really need both of them, especially since I'm mostly a hobbyist - and space is at a premium in my little bedroom studio. But when I try to decide which I'd part with, I just can't decide.
In comes the Trigon! As soon as I found out the filter is switchable between 2 and 4 pole modes, a light bulb went off: could this single synth essentially replace both of the others? I know it's a different filter from either, and again, it too has it's own sound, but c'mon. They're all kinda varied flavors of the same basic form factor and architecture.
Could the 4 pole mode basically cover P6 territory while the 2 pole mode covers OB 6 territory, recognizing there are some differences?
I know not many have had their hands on one yet since it's so new, but if anyone has experience with all three, or if you want to just join in the totally unsubstantiated conjecture like I'm doing, I'd love to hear from you. 
One thing to consider is sometimes there is a huge amount of airtime given to the differences between synths - the areas they can go to which other synths can't - rather than shared ground. Even with the P6 and OB6, of course there are differences and if you dive full into 'Oberheim' territory on the OB6 you will find things the P6 can't do, and vice versa. However, the shared ground between them is quote huge - even if their filters are so different. So I would take that into consideration - and try to explore how much shared ground there is between the Trigon and those two. I should imagine that the Trigon actually has even more shared ground with both the P6 and the OB6 (due to the filter options) whilst also sharing much of the same architecture. Further it has that third oscillator which gives you even more options too. So in summary - are there aspects really specific to the P6 and OB6 which you would really miss which you perhps couldn't cover with the Trigon?
Also, if you are struggling for space going down to one synth could be the answer. I know some folk who really believe that you don't need more than one polysynth, and that you can get 99% of what you want through good programming, effects etc. So if you can believe you can do that then why not?
Another thing to think about too - would you miss having two synths playing in parallel? Is this something you utilise a lot, or do you tend to record etc with one synth at a time? If the former you may want to think about that.
Lastly, I will say what I always say to anyone considering buying a new synth - if you do get the chance to try one, does it inspire you? Does it 'feel' right? This is not something that is important to everyone mind - but for me personally I tend to be much more creative with instruments (not just synths) which feel right somehow. That's really woolly I know, and probably not well described - but I hope it makes sense in some way!