I would recommend having your pups meet on neutral territory, like a quiet park. Have all dogs leashed and muzzled (very important when dealing with a dog of unknown background), and take two dogs out at a time.

For example, let Tucker approach Syloh, allowing her to go at her own pace. Treat or praise her for any signs of relaxing or curiosity. If she doesn't attempt to approach Syloh, give her a couple treats and walk towards Syloh. Keep walking and keep treating until either she gains interest in Syloh or you're close enough now and she's not growling. Now, take Syloh away and bring out Cozy. Let Tucker meet at her own pace and, again, treat and praise for any good behavior. Take Cozy away again, and play with Tucker. Next, bring out Cozy and Syloh and have them sit or lay down calmly. Let Tucker approach them and treat and praise for good behavior. You can also try going for a walk with all three or just playing with Tucker.

When you go home, always supervise Tucker when she's with your other two, and crate her when you feel like she's being overwhelmed. Eventually, she'll come out of her shell, but it does take time. Good luck!