I'm pretty sure that MHP3rdG would have sold a lot more based on the sales of 2ndG,
Infinite growth! Just think what MHP6thG would've sold! 5 billoion!
Electroplankton: It took quite some time for MPH3rd to reach 4.5 million copies sold, so no one expected TriG to match this at day one.
No one expected it to reach 4.5 million, at all. It's exceeded Capcom's expectations for the game, and it'll probably end up close to MHP2's sales numbers. Sure a MHP3rdG on PSP would've sold more than MH3G (but possibly less than MHP3rd), but as has been beaten to death numerous times, you can't just keep making games for the same system forever. Eventually there needs to be a transition to a new platform, and that will necessitate smaller sales numbers for multi-million selling series, at least early on. Smaller series like Miku (big compared to many, but not multi-million) have an easier time selling close to their predecessors, and then there are rabid fanbases like MGS (roughly the same LTD for most main entries, regardless of platform/userbase).
Miku didn't do badly since it sold less than its predecessors on PSP, though. It was on Vita which has been tanking pretty much all year, so no one reasonably expected it to do better than the previous entries. It did pretty good considering the Vita's current state. Might have done better elsewhere, but they do have a PS3 port coming, so that should help. However, trying to tie together Miku's performance and MH3G's performance to make Miku's performance seem better seems like way too much of a stretch.