Harmonix said that the PS3 version is done, and that the company is submitting the game to Sony's certification process today. The studio is hoping to be able to release the game in "early April," although that depends on how the process goes.
It's been a long time coming. Harmonix launched its Kickstarter campaign for Amplitude in May 2014, and planned at the time to release the game on PS3 and PlayStation 4 in March 2015. In January 2015, the studio pushed back the launch to the summer. Then, in July, Harmonix announced that it would release Amplitude in December 2015 for certain Kickstarter backers and in January 2016 for everyone else — and that the PS3 version would arrive "shortly behind" the PS4 version.
Amplitude did indeed launch on PS4 in those windows; the public debut came Jan. 5. This is the first time Harmonix has provided an update on the PS3 version's timing since mid-January, when the studio said in a Reddit AMA that it was targeting a spring 2016 release.
Harmonix also provided an update today on Amplitude's Kickstarter rewards, some of which have still not yet been delivered to backers. The outstanding physical rewards "will be ready slightly after the PS3 version hits, given current estimates," said Harmonix. That includes the vinyl record of the game's soundtrack, which was a reward for the $200 tier of the Kickstarter campaign and should be shipped to backers by "mid- to late-April." As for the remaining digital reward — an art book from the $70 reward tier — Harmonix is sending it to backers today.