If I can no longer pay for my vehicle (it's being financed) can I just return it to the Lender? Will I still be liable for the remaining balance (it's only about $3k)?

Yes and yes. If you return a vehicle (voluntary reposession), the vehicle is then sold, usually at auction. Any proceeds from the sale are appliecd to the outstanding balance. Whatever balance isn't covered you will stilll owe.

Biggest mistake of my life was returning my vehicle, they sold it at auction for 1500, I still owed 10,000 on it once they applied the fees they incurred in selling it, I basically just gave a car away plus still owe on it. If I were you talk to the financing company see if there is anything they can do to lower your payments. If I had it to do all over again I would have taken a second job or anything else just to pay that stupid car off. The trouble it has caused me now is so much worse. See if someone can take over your payments, before you give it back make sure it's your last resort.