If you are starting out with fish that are 3 inch you should be able to tell the sexes apart.
Green terrors can be sexed quite early on.Young males will normally be showing signs of dominating behaviour if you watch them long enough.They will be a brighter colour than the females also at an early age.
They will also pair up and breed when only around 100 mm long.
Here are some I had.They are around 100 mm at this stage.You can clearly tell the male from the females.
Male is at the back near the bottom in this photo
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I raised a group if 5 females and 1 male in a 7x2x2.It worked very well.
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Your tank is only big enough for a pair and you may find your male is hard on your female at times.I would make sure you have plenty of places she can retreat to if the male gets too aggressive.
As said already a group of six juveniles is a good number to start with.
Be ready to move the other fish once a pair forms. Aggressive behaviour can form very quickly so have a plan ready for when this happens.
Good luck.