I moved your post to the bonding- section.
very young rabbits as a rule get along well. At 3 months, the girl you have is about to start puberty – so make extra-extra sure the new bun you’re getting really is a she, or you’ll get baby bunnies sooner or later. It’s often difficult to sex young rabbits, so beware!

When the new girl comes home, it’s good if she gets a little time for herself separate from the bunny you already have. She’ll need to get used to her new home.

Hi! First, both buns will need to be fixed before they can physically interact. Otherwise, the hormones will cause vicious fights between them. The spaying will have to be performed by a rabbit savvy vet, then after they heal you can start the bonding process. In the meantime, I think you can start prebonding? (ie. introducing them to each others’ scent, visual introductions) But it’s also important to give them both time to settle into their new home and get comfortable beforehand .