RE: OVA3 to GXP timeline.
From the wikia (so take with a pinch of salt), apparently the punishment was modified to a year. Given Misao is in the Galaxy Army in GXP, there could be many reasons for this (besides the 'what is a year?' question, there is tapped to infiltrate the GA faction partnering with Shank, working his butt off, time travel, earning it by being Seina's clean-up crew...)
So as far as I know, GXP was only a year after OVA3. Given the relative ages of those seen in the two series, it does seem likely.
From the wikia (so take with a pinch of salt), apparently the punishment was modified to a year. Given Misao is in the Galaxy Army in GXP, there could be many reasons for this (besides the 'what is a year?' question, there is tapped to infiltrate the GA faction partnering with Shank, working his butt off, time travel, earning it by being Seina's clean-up crew...)
So as far as I know, GXP was only a year after OVA3. Given the relative ages of those seen in the two series, it does seem likely.