Lucky you to be able to get a T3 at this stage.. I got mine less than 3 months back and now there's none to get at all in this country (I wanted to get a spare when I realized P1 stopped the production.. this is the most useful PDA I've ever tried, and I just more and more appreciate the incredible small form factor, all due to the slider. Too bad stopping now, P1!)
As for navigation: I don't know how/if the USB GPS unit can be used, sorry. As for TomTom, it's great. I own TomTom Navigator for Palm, and TomTom's Bluetooth GPS unit. I'm a bit uncertain about what exactly you mean by "tomtom navigator europe 3", as that is not a description I immediately recognize.. software, or maps? Be aware that the product called "TomTom Navigator 3" isn't for Palm, it's for PPC. The software product codes for Palm versions start with "1B" while the PPC versions start with "1C" (as in "1B50.092" which is "TomTom Navigator for Palm, Nordic version, Bluetooth". The 1B00.092 version is the wired (non-BT) version, and so on. The TT3/PPC versions would be the same, but starting with 1C)
It's a possibility that your USB GPS can't be used, however there are a couple of utilities you can download to start checking it out (edit: Uh, you said you didn't have your T3 yet.. but anyway these are great additions to TomTom)
http://www.cetusgps.dk/ (the beta version also works with BT GPS devices)
http://core.de/~coto/projects/cotogps/
Both are free GPS programs for Palm.
-Tor