Hi there. I assume you have tried having one person hold your piggie while you try to check him/her out? I never believe a male is a male until I get the penis to pop out. If this piggie is 3 months old, you should be able to press down and get the penis to pop out if it's actually a male.
Feeling for the ridge is a very good suggestion. Also, look at the genitals. Do you see a definite opening? If you spread it apart does it open? That would be a female. A male would have a sac that would sort of roll back rather than open - sort of like a pair of rolled up socks is the best way I can describe it.
You're better off to spend the money on a vet to tell you for sure (assuming you have a cavy savy vet!) than to accidently pair up a male with a female and end up with babies!!
If you do end up having a male, don't worry too much about intros. You would be better off with either an older male (past puberty) or a baby. Babies are usually well tolerated, but remember the baby will go through puberty eventually too. If it was me, I would go for an older male so the only one you have to worry about going through puberty is the one you have now.
You also have the option to try to find a spayed female (if you do have a boy on your hands) or to neuter your boy and get him a female friend. You can't decide anything until you know for sure if you have a boy or a girl though. Good luck and let us know what you decide to do.