Well, I happen to have developed a few thousand rolls of Tri-X in HC-110 Dilution B. Seriously. I think it's a great developer, but if you are mixing it up using concentrated developer and then water, it is absolutely *imperative* that you rinse the measuring graduate with the same amount of water, multiple times, and do it the same,exact way, each and every tank of film.
My preferred time with HC-110 and 2 rolls of film was 7 minutes.
Take a look at
Kodak HC-110 Developer - Unofficial Resource Page
which is an HC-110 fan page,and note that the author openly writes that he believes that Kodak's "new" suggested time for Tri-X is the WRONG TIME....and I would have to agree. A suggested time of 3.5 minutes at 20 degrees Centrigrade/68 degrees Farenheit is simply ludicrous! Your bad results show that the time yields crummy-looking negatives that do not develop anywhere near the film speed or contrast desired...I looked at your sample pics--TOTALLY flat negatives. The other thing leading to the idea that the Kodak times are wrong is the fact that development times of 3.5 minutes are *NOTORIOUSLY* unreliable and not-repeatable. For over 15 years, Kodak literature specifically warned against short developing times!
A second factor many people are totally unaware of is the accuracy of their own thermometer; are you using a quality process thermomenter, or a simple darkroom thermometer? I used to do lab tech work, processing 4, 8,12,or 16 rolls of Tri-X in HC-110 dilution B every night. It is common for stainles steel dial thermometers to read as much as 3 to 3.5 degrees differently than one another, and using an ultra-short dilution time of 3.5 minutes seems patently ludicrous to me. When I read that the suggested time for Tri-X in HC-110 B was 3.5 minutes, I did not believe what I was reading....it used to be more like 6:30 to 7:00 at 20C. Development times for films is a highly individualized,personalized matter than incorporates many things: Exposure Index you exposed at, your light meter, your camera, how you meter (Zone System with a calibrated spot meter or wave-o-meter-and-just-shoot),as well as the developer's exact strength, thermometer accuracy, and agitation method and agitation time/soak time.
It's a lot like pizza cooking--times are 'suggested starting times', not written in stone. But 3.5 minutes....something is very,very suspicious about that time...it is highly suspect.
Agitation using HC-10 can be done 5 seconds every 30 seconds, or 10 second on the minute, using a rolling-pin agitation method, made famous by Bill Pierce in the early 1970's. Clearly, the 3.5 minute time period for developing any B&W rollfilm is, well, just idiotic. Simply no other way to put it,and in fact the older data sheets warned of the dangers of very short developing times--uneven,streaky development, inadequate emulsion speed,and extremely unpredictable and un-repeatable results.
Obviously, experienced darkroom workers dispute Kodak's published 3.5 minute time with dilution B HC-110. It's highly concentrated; 6 millilitres is enough to soup a whole roll of film, and so exact dilution and strength are paramount if mixing it one-shot or by the gallon. If the negs look thin, you need to develop longer, no matter what Kodak says. Especially if using a fixed shutter speed camera like a Holga or Diana.