No sound third octave on korg trinity pro x 88 keys

The analog cx3 uses old school organ technology where a single oscillator for each scale note is divided to put it in the correct octave. Its likley the c# oscillator needs to be replaced. This is a job for a tech rather than being user serviceable.

Here's a video showing you exactly what to do! https://www.youtube.com/watch?v=HXFN0LymD0Q

Try this link. They still have the on the Korg website.
https://cdn.korg.com/us/support/download/files/f95e62dfd278a93f094a2b7d67de1e29.pdf?response-content-disposition=inline%3Bfilename%2A%3DUTF-8%27%27TRINITY_BG_E.pdf&response-content-type=application%2Fpdf%3B

You have a dislodged/dirty/broken ribbon connector coming from the keyboard. If you're not experienced in the disassembly and repair of your instrument, take it to a service center.

Unfortunately, the manual on
http://www.korg.com/us/products/tuners/ga_40/

indicates that 12 string is not supported.

The owners manual can be downloaded at
http://i.korg.com/uploads/Support/GA40_EFG1_633661472559800000.pdf

The YouTube tutorial is at
https://www.youtube.com/watch?v=Of4CRajvq0E

with a 12 string, you can often tune the octave until the tuner sees the matching note. It won't see the correct string number, but an A note is an A note, whether or not it's an octave higher.

Hope that this helps.

clean with dust off and a damp rag. might have stuff stuck in the keys

Vera hi
to fix this problem you need open where are the contact board keys and clean or change the rubber contact

1. Power-off the PS60.
2. While holding down the four PERFORMANCE buttons and the OCTAVE DOWN button, power- on the PS60. The PS60 will be initialized, and the factory-set data will be written into internal memory. The display will indicate "Initializing..." while data is being written. Never turn off the power while this indication is shown.

Manual available here:
http://www.korg.co.uk/downloads/ps60/docs/PS60_OM_E1.pdf

This sounds like a power supply issue... Since the unit is new, make use of the warranty for repair or replacement.

You will likely have to double that budget to get anything decent... but look for a Yamaha in the PSR series (61 keys minimum). To get a full 88 key, you might have to be up in the $400 range for anything really decent.