I'm a big fan of the MotorStorm series. So count me among those who take Sony Computer Entertainment Europe's recent (as in, Friday) registry of a MotorStorm 3 domain as good news, not just a company hedging for future plans.
The domain redirects you back to the Official PlayStation web site for the UK (the country selector if you haven't picked that region.) WHOIS records show it went active on Jan. 19.
MotorStorm's sequels have not been numbered - Pacific Rift was in 2008, Arctic Edge for the PSP and PS2 was last year - so this tells us nothing about the game. But if there's a game on the way this would roughly follow the two-year cycle for MotorStorm on the PS3.
I've emailed a Sony contact but I don't expect them to say much.
MotorStorm 3 Domain Name Registered by Sony Europe [The Gaming Liberty, thanks Shane W.]