Hydraulic pressure is applied to the 3rd clutch, then the 3rd clutch engages the countershaft 3rd gear with the mainshaft.
The mainshaft 3rd gear drives the countershaft 3rd gear and the countershaft.
Power is transmitted to the final drive gear, which in turn drives the final driven gear.
NOTE:
Hydraulic pressure is applied to the 1st clutch, but since the rotation speed of 3rd gear exceeds that of 1st gear, power from 1st gear is cut off at the one-way clutch.
The illustration shows the 7-position transmission; 6-position does not have the sub-shaft and related parts.
or 
Position in 3rd gear
