Six-speed manual gearboxes often have gears one, two and three sharing a shaft inside the gearbox. The fact that you can select all those gears but not have any drive in them suggests that something on that shaft is failing to mesh with either the countershaft or the output shaft.

Does the gearbox make any weird noises when you select any of the first three gears? Is there any oil leaking from the gearbox casing? It could be anything from a stripped gear or spline to a failed bearing. Either way, I think it’s probably a fundamental mechanical failure and you’re looking at a gearbox rebuild or replacement. At least if the top three gears are working, everything downstream of the gearbox seems to be okay.