how to get back lost contacts on my samsung s3?

Um.... In fact, I think you had better backup data for your GS3 before rooting. Generally speaking, rooting won't wipe data from phone. Anyway, if you want to recover lost data from Samsung Galaxy S3, you had better stop using your phone to prevent data overwritting. And then, you can try to use some Android data recovery software such as Tenorshare Android Data Recovery, I ever used it to recover data from my Samsung Galaxy S4, Good luck!

Actually, all you lost data are still on your Android phone or SD card for Android.You can’t find the lost files back without recovery tool. The only way is to use a software so that you can get back SMS,Contacts or Photo&Video from your Android mobile Phone.
Edit [Moderator]: Link removed
1. Directly recover deleted SMS text messages and contacts from Android phone
2. Retrieve photos,gallery and video lost due to deleting, restoring factory settings, flashing ROM, rooting, etc., from SD cards inside Android devices
3. Preview & selectively recover messages, contacts and photos before recovery
4. Support multiple Android phones and tablets, Samsung, HTC, LG, Motorola, Sony, Huawei,ZTE,ect.

I assume you mean you lost your phone's numbers/emails and so on? Just check your Google accounts sync. Go to Settings > Accounts > Google and make sure that your contacts are set up to sync properly.Then you will restore your android contacts.

By the way, this should be posted elsewhere, as this portion of the forum is for computer discussion.