Let me make an off-topic comment first.
TH2Go does not necessarily go against large screen HDTV. "vs" is not the only option. The two can go along well with each other. In other words, you can have the best of both worlds if you use three large screen HDTVs with a TH2Go device. :D
As to your case, if it is purely for gaming, I will choose 3 x 24". But if you want to watch Blu-ray movies occasionally, I'd choose the 42".
I have both anyway. and can switch with ease
(Th2g on DVI 1 and HDTV on DVI 2 of my video card)
I think it's more Th2Go vs HDTV. sure u can have 3 HDTV on a TH2G but that's not an option unless you're really rich and have alot of space in the house.
the huge bezel around hdtv would be a pain to deal with too.
with triple 20" monitor it's a good productivity tool. but with 3x 42" HDTV it's pretty much overkill and I see no use for it other than gaming. and its not cheap.
btw: this weekend I put in about 7 hours playing UT2004, ut3, Call of duty 4 and doom3. the situation and environment awareness with the TH2G is awesome.