2010 4WD 3L, 185,000Km "falls" out of 3rd

[NZFRANK]

Hi,
With engine off, clutch in, engage 3rd gear. The lever returns to neutral under gravity. When driving, normally falls out of third, sometime stays in third for a while. No load, Load and overrun make no difference. All other gears stay in.
I believe previous owner held onto gear knob, as it is very shiny.
Anybody know what the fix is?
Thanks
Frank

[campbeam]

Following quote is from this URL: 

https://community.cartalk.com/t/pops-out-of-third-gear/80757 

"In most cases this is caused by wear of the 3/4 shift fork and/or wear of the 3/4 synchronizer assembly; most notably the synchronizer sleeve. The only fix is to go inside the transmission." 

Too many years since my last manual vehicle but I learnt/knew not to drive with your hand resting on the gear lever. 

[CONROD]

3 plugs on the side of the transmission, These are detent plugs, inside is a short spring and a ball bearing, easy to check, just undo, be bloody careful not to lose spring or ball, Otherwise buggered syncro ring