When l shift into 3rd it sometimes goes in fine but sometimes grinds.
what should l do?

the synchronizes may be bad in the transmission for third gear,that is not uncommon for that too happen on those,dodge did have some problems from a lot of their transmissions,try letting a transmission shop test drive it and see what they say ,its free then this way you,ll have a hands on opinion on what might be causing that to happen,good luck.