The quickest way is this- On each of the front and rear mechanisms (Derailers) are two limit screws. They are marked L(low) and H (high). Front mech- Put the mech onto the middle chainring ,(Using your gear changer), and screw in the limit screws until you feel resistance. They will keep the bike on this middle chainring only. You now have 6 gears. Rear Mech- Put the rear mech on the third largest cog ,(Using your gear changer), and screw the L screw in until you feel resistance. Put the mech on the second smallest cog ,(Using your gear changer),and adjust the H screw in until you feel resistance. You now have 3 gears. This may not be exactly what you wanted but its quick cheap. A bike shop could do this in about four minutes. Anyway sorry for the rambling explanation and good luck.