Why does my car keep slipping out of 3rd gear?

I own a 2011 BMW 118d and recently the gearbox kicks me out of third gear but all gears are working fine with no issues. I am just wondering what it could be before I start spending money on unnecessary...

Does my manual MINI need a gearbox service?

Does my MINI Countryman manual 2013 1.6D need a gearbox service?

My car broke down a month after its MoT - should this have been picked up?

My Ford B-MAX was MoTd in May with two advisories for tyres and dust caps. It then broke down the first week July, the gearbox had a serious oil leak which I have been told should have been picked up on...