How do I charge a 6V, Lead Acid, 4.5 Ah battery using a 3W solar panel?
Because the solar panel puts so little power compared to the size of the battery, and lead-acid batteries are rather forgiving, all you need is a Schottky diode.

Connect the diode in series with the solar panel, cathode to battery +, anode to panel + output.  Tie together the panel - and the battery -.  That's all you need.  The float charge level of the battery will be around 6.8 V, but the solar panel is too wussy to deliver enough current at that voltage to cause any harm.  If you're really worried about overcharging, add a shunt regulator across the battery set to 7 V or so.