To change AC to DC you need a rectifier. The best solution is to use a bridge which can be made from four diodes or a bridge rectifier which is a square block with four connections, two for AC and positive a negative output. You may need a heat sink if running at the full 3A.
To limit the current to a safe level, you could place a bulb in series. I have a tame mechanic friend who saves me headlight bulbs which have one broken filament. An ampere meter whould be advantageous as would a voltmeter. Car alternators are usually set to limit charge to 13.8 to 14V for lead/acid batteries.