If you want to test it yourself, here is the link to download TRP3 alpha.
I've also posted on the WRA forum "TRP3 who is making the switch" post, and there might be some added information there as well from others
Here is what I found after 5min or so of playing around with the new alpha. Thanks go to Keen, who willingly stood there and let me test the compatibility with TRP2.
"I've made a point to be familiar with TRP2, and have written guides about how to use the majority of the functions. My guild, Sentinel, also encourages TRP2 over MRP because we use items in our RP.
Today, I had a fellow TRP2 guildie stand there as I tested out some different aspects of TRP3. The main thing that I noticed that some features don't seem to communicate well in the alpha, and others are missing completely (no background info in TRP3, no misc RP info in TRP2, etc.) I understand this is just the alpha, but I'd like to talk about some of the changes between TRP2 and TRP3, for those curious.
To start off, the interface for TRP3 is much different. You still have the basic bar (show/hide helm, IC/OOC toggle, etc), but it's greatly reduced; no more items, quests, stashes, etc in the alpha. It remains to be seen if those will be added in later, or if we'll have to use GHI to make up for this loss.

The main interface is streamlined and follows the basic look of the blizzUI. You can also make more than one profile per toon, something that TRP2 didn't allow before (Alesune now has her Sentinel profile, and a trolltastic flightform AFK one).

You're given the same basic character directory sheet with name/title, birthplace, eye color, etc. You also have a place for "About", which I'm assuming is the physical description AND the background combined. It seems from my guildie and my testing that only TRP2 descriptions will appear here, and not their history, leaving you completely in the dark if they have notes there about their character's background.

TRP3 also has a new 'misc' area, where you can put in clear terms what kind of RP duel you like (PVP, dice rolls, etc), if you're okay with injuries or romance, or even if you'd let someone ICly kill off your character. This new feature is for TRP3 only, but covers a lot of the gray area you might have when starting new RP with someone you don't know. In the misc section is also an 'at first glance' place, where you can make up to five icons that seem similar to states. This didn't seem to be communicating through the different addons, as I couldn't see her TRP2 states, and she couldn't see my 'at a glance' state-like things.

TRP3 also makes it pretty clear if you've seen someone and RPed with them before, because when you click on them for the first time it comes up with a button as to whether they are your friend, business only, neutral, unfriendly, family, etc. That makes it extremely easy to sort through profiles that are saved into your directory. It's also helpful if you play more than one character, so you know exactly how your current char feels rather than wondering if you saw them on Toon A or Toon B."
* I'd also like to add that there is a "this realm only" box. Perhaps we might see xrealmable TRP3s in the future? *
As of now, the compatibility between TRP2 and TRP3 is a bit off, but that's understandable; TRP3 is still in alpha. If at all possible, I personally think we should keep TRP2 profiles until the point the majority of the guild agrees to switch over, for the simple fact the alpha doesn't communicate fully yet. That isn't to say you can't and shouldn't mess around with TRP3 and get to know some of the new buttons!
Just remember, if you want your TRP2 description in your TRP3 profile, you need to copy/paste it into a word document, because you can't have them open at the same time or import your TRP2 into TRP3 in the alpha.