No, when they trademark something like this they need to file it with intent to actually use it or else their claim to the trademark expires. In other words, they only would've done this if they knew they were gonna make it.
No need to be on the fence about this, Half Life 3 is coming!
There's just no formal announcement yet.
Just look at corporatesteve's link, that's directly from Valve internal, LFD3 and Source 2 are also being worked on!
With Steam machines being worked on full swing as well they're really putting all that sales money to good use now! xD